Pan-cortex-data-lake

- Reversed the `access_token` lookup order in get_credentials() method.
- Added `_resolve_credential()` to `access_token()` property method to support token caching.
- Now comparing passed `access_token` in `refresh()` method to value returned by property method.

- Added pancloud.adapters to find_packages include list to resolve issue with PyPI package.

- Added requirements.txt to MANIFEST.in to address build issues with PyPI package.
- Fixed issue in README.rst that prevented PyPI from properly rendering.

- Updated README.rst
- Updated RTD API Reference.
- Added `pancloud` to requirements_dev.txt
- Added docstrings to `Credentials` property methods.
- Changed logging xpoll() to return entire log entry instead of just `_source` dictionary.
- Added `Credentials` Storage Adapter feature and moved `TinyDB` code to `tinydb_adapter.py`, the default storage adapter.
- Automatically carry `queryId` from `logging --query` response to `--id` in subsequent `--poll`, `--xpoll` and `--delete` in `summit.py`.
- Various bug fixes and improvements to `summit.py`.
- Added support for caching `access_token` in credentials store.
- Added `write()` method to `LoggingService` class to support writing logs.
- Fixed issues with `Credentials` `get_authorization_url` and `fetch_tokens` methods.
- Added`logging_write.py` to examples.

- Updated logging `xpoll()` to reflect behavior of current API.
- Added `credentials.py` module to support OAuth2 operations.
- Added `auto\_refresh` and `auto_retry` support to `HTTPClient()`.
- Added `PartialCredentialsError` exception to handle cases where incomplete credentials are passed to `Credentials`.
- Removed extraneous dependencies, e.g. pyopenssl.
- Cleaned up requirements.txt and PipLock files.
- Added `TinyDB` package for reading/writing `credentials.json` file.
- Now raising `PanCloudError` for `enforce_json` errors.
- Removed logging `poll_all()` method and example script.
- Now checking if `sequence_no` is `None` before incrementing in logging `iter_poll()` method.
- Added event `xpoll()` method to support iterating through events.
- Introducing `summit.py`, a command-line interface for `pancloud`.
- Added `credentials` support to `summit.py` to enable writing `credentials.json` file and passing `Credentials` with requests.
- Updated example scripts library to incorporate new credentials feature.

- Overhauled and updated RTDs
- Updated `__init__.py` to allow package-level imports
- Fixed issue where starrifying authorization token in `__repr__` overrode value in request header.