Introduce 'retry' option

Add a 'retry' option which allows to configure how many
retries pip should make before giving up on HTTP request.

When the retries count is specified by user, its value is
passed to HTTPAdapter from requests which handles all
the underlying operations.
Fix minor typo
Documentation for --exists-action option

Fixes #1458
Fix "it's"/"its" typo spotted by @kraai
Ensure wheel-Version is compatible with pip
Fix tests to expect installation or not
Explain futurewheel version numbers
Merge pull request #1568 from coagulant/feature/document-exists-action

Documentation for --exists-action option
Small cleanup, use email.parser.Parser
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
Merge pull request #1567 from ajdm/minortypofix

Fix minor typo
Use find_on_path, extract compatibility check
Add unit test for check_compatibility
Python 3 compatibility
Remove commented line
Fix test logic, tiny error / import cleanup