Documentation for --exists-action option

Fixes #1458
Merge pull request #1568 from coagulant/feature/document-exists-action

Documentation for --exists-action option
This is no longer in use
Merge pull request #1575 from dstufft/remove-unused-constant

Remove an unused constant
Modify __main__.py to allow running from wheel
Merge pull request #1576 from pfmoore/executable_wheel

Modify __main__.py to allow running from wheel
Merge branch 'develop' of https://github.com/pypa/pip into test-wheel-version
Use find_on_path, extract compatibility check
Add unit test for check_compatibility