allow for keyword args in init_repository

This implements five ways to init a repo:

    import pygit2 as git
    git.init_repository("/tmp/foo/")
    git.init_repository("/tmp/foo/", True)
    git.init_repository("/tmp/foo/", False)
    git.init_repository("/tmp/foo/", bare=True)
    git.init_repository("/tmp/foo/", bare=False)

Since one is still able to pass the bare as a positional argument, this change
is perfectly backwards compatible. All existing code will continue to work.
tests for keyword arg init_repository
document new ways to init a repo
Fix build after latest changes in libgit2
Merge remote-tracking branch 'esc/feature/init_repository_keywordargs'