friendlier repo.init_at API

is_bare is now optional and defaults to false, same as git init. It's also
parsed as normal Ruby truthy value instead of type checked for True
or False, so more readable calls like this can be made

   Rugged::Repository.init_at('~/repository', :bare)
Merge pull request #171 from nvloff/friendly_init_at

friendlier repo.init_at API