Captainhook

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- Replace sed usage with more pythonic cross-platform approach for installing
using a virtualenv
- Fixes for flake8 version 3

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- added pytest coverage checker.
- forced isort to be at least 4.1.1 to disable output.
- added ``frosted`` check
- made non-compatibility with python<2.7 explicit.
- fixed a bug with blocked branches on py3.
- fixed bug when captainhook section is missing.
- flake8 now obeys tox.ini files in the root of the directory.

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- Add checker for merge marks.
- Add block_branches checker to prevent commits to eg. master.
- Use REQUIRED_FILES for isort.
- Add REQUIRED_FILES variable for checkers - allowing them to copy additional
files if present.

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- Don't install checkers if the hook can't be installed.
- Add isort checker for import checks.

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- Add dependency on docopt for easier argument parsing.
- Change install to be an explicit subcommand: `captainhook install`
- Add a command to run checkers against uncommitted code: `captainhook run`
- All checkers now take a list of files to check.
- Only use `file` to determine pythonic nature if there is no extension.
- Use py2to3 to analyze python files' py3 compatibility.

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- Analyze the versions of the file to be committed.
- Fix ``bash`` unicode decode errors.
- Add behave tests for pdb.
- Add option to run hooks from current virtualenv.
- Add --version option.
- Add option to filter out files in pdb_checker.
- Add travis.yml file.
- Add tests for pdb checker.
- Add __version__ to checkers at install time.
- Add a grep checker.