Azkaban

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* Add flow extension.
* Improve `urllib3` warning suppression.

Breaking changes:

* Change behavior of `Config.get_option` method to not preprend the `"default"`
prefix when looking up the option.

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* Display progress percentage during project upload.
* Added `log` CLI command to view flow and job execution logs.
* Improved logging and added default file handler for the CLIs.
* AzkabanPig now waits when the job is in a preparing stage, rather than
erroring out (because it is unable to find the logs).
* Cleaner error message when the Azkaban server is down (i.e. on 500 response).
* Changed default `--project` CLI option to support both `jobs.py` and `jobs/`
as defaults.
* Added `--option` flag to `build` to allow overriding of project properties.
* Added `schedule` command for workflows.
* Added `schedule_workflow`, `unschedule_workflow`, `get_schedule` methods for
working with azkaban scheduling.
* Added `get_workflow_executions`, and `get_running_workflows` methods.
* Enhanced URL parsing support from `.azkabanrc`.

Breaking changes:

* Removed `create` and `delete` CLI commands.
* Changed `--options` handling for `info` command (and renamed it to `--option`).
* Renamed `--skip` option for running flow to `--bounce`

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* Added ability to pass runtime properties when executing a workflow.
* Keyword arguments passed to the `Project.add_job` method will be forwarded to the
`Job.on_add` handler. In the case of a project merge, the handler gets the
merged project as single keyword argument.
* Added `Execution.logs` and `Execution.start` methods to respectively view the
global execution logs and start a new execution from an existion session.
* `Job.build` now writes to standard out when no path is specified.
* `Project.jobs` now returns a dictionary of actual `Job` instances.

Breaking changes:

* Removed the `Job.on_build` method.
* Removed the effect of the return value of the `Job.on_add` handler. This lead to
overly complex archive file generation.
* To regain some consistency, the `PigJob` class doesn't expect a path to a pig
script as first parameter anymore. Instead it requires the `'pig.script'`
option to be specified.

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* Added `Job.join_options` and `Job.join_prefix` helper methods.
* `azkaban --files` now also outputs the target filepaths.

Breaking changes:

* The return value of the `Job.on_add` and `Job.on_build` handlers now
respectively determines whether the job will be added or built. The
default implementations accordingly now return `True`.

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* Added project `properties` to set project level default parameters.
* Added `overwrite` option to `Project.add_file` method.

Breaking changes:

* When adding files to a project where a `root` was specified, the default
archive path will be path relative to the project root. Adding files
above the project root without specifying an archive path will now raise
an error.
* Merging projects is now only allowed for projects with the same root.

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* Added `azkabanpig` extension. It includes the `PigJob` class and
a new entry point that allows for easy running of pig scripts.
* Made `--alias` and `--url` options optional (indeed).
* Enabled session refresh after initialization (useful for long running
jobs, for example with the `azkabanpig` extension).

Breaking changes:

* `~/.azkabanrc` format changed and isn't compatible with the previous
one. It now allows for specifying custom defaults for some options.
* `Project.main` method is not supported anymore.
* `azkaban list` and `azkaban view` are now combined into `azkaban info`.