Implement support for getting repo by id

This allows the Github class to retrieve repos by either their full name
or id. If the user passes in an id for a repo which doesn't exist or in which
they don't have sufficient permissions, an UnknownObjectException will
be raised. This addresses issue #245.
v3 changes gist.user to gist.owner
Merge branch 'develop' of https://github.com/tylertreat/PyGithub into develop
Acknowledge some lack of test coverage
Prepare merge of #247

- refactor Gists tests
- re-add Gist.user to not break client code (value as been None for a while, but let's not remove the attribute)
Merge branch 'pr247' into develop
pep8

This is strange, maybe Cygwin's version is stricter.
No time to investigate.
Publish version 1.25.0