Browserstacker

* Changed commands names.
* Fixed logger setup.
* Initial support for retries (3).
* Dropped sessions support due auth complications.

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* Added verbose output option (9).
* Added filtration for `list_browsers` (10).

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* Added `make_screenshots` shortcut (7).
* Added default_browser option (8).

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* Used single `requests` session for all calls (1).
* Package is uploaded to PyPI (2).
* Documentation added (6).

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* Initial release.