Branches has its own class, adding rename and merge methods

[thibaultduponchelle]

Hello 😃

I propose to add these methods rename and merge and doing this change required to reorganize the code to add a new Pithub::Repos::Branches (the same way is Pithub::Repos::Commits and friends).

[oalders]

Thanks, @thibaultduponchelle! Before we review, let's get the build passing.

#   Failed test 'pod coverage for Pithub::Repos::Branches'
#   at xt/author/pod-coverage.t line 38.
# Coverage for Pithub::Repos::Branches is 25.0%, with 3 naked subroutines:
#   get
#   merge
#   rename

[thibaultduponchelle]

Ok 😃 It should be fine now

[thibaultduponchelle]

Closed because #220