Dsub

This release includes incremental progress on the Google Genomics Pipelines v2 provider and general refactoring to enable this upcoming feature.

Other user visible changes include:

* Support for empty `--inputs` and `--outputs` from the command line and task files.
* `dstat` now includes provider attributes, such as compute zone and machine type.

This release includes new documentation, bug fixes, and test optimizations. A highlight of the release is the Split and Process demo contributed by JP. In addition the following new features and bug fixes are present.

* Support for europe-west4 and northamerica-northeast1 regions
* Add support for empty environmental variables
* Require Python 2 during install
* Multiple test optimizations.

- Allow use of `--skip` and `--tasks` flags
- Fix rare edge case causing incorrect ordering of jobs returned from dstat
- Allow specification of upper-bound for job create-time
- A number of small updates to the (still experimental) Python API

This release includes several improvements:
- Fix `dstat` job/task ordering for `GoogleJobProvider` and `LocalJobProvider`
- Fix duplication of `dstat` output if duplicate parameters are passed in
- Added new GCE zones for `asia-south1` region

This release includes several improvements:

* Added support for GPU accelerators!
* Add start-time to `dstat` full output.
* Update getting-started documentation.

This release includes several improvements and bug fixes:

Bug fixes:
* `ddel` validates provider-specific arguments (see bug 87)

Internal improvements:
* Improved use of the `--age` flag allowing some tests to run faster.
* Refactored local provider to pull out `runner.sh`.