Adjust branch option to specify the branch to collect statistics on

Change:
 * The function of the branch CLI flag to specify the specific branch
   we want to collect statistics on. This differs from the original
   function since we use to be able to collect all commits on a set of
   branches. Rugged only allows us to walk through from the HEAD of one
   branch at a time (we might be able to expand on this by doing
   multiple walks from the HEAD of each branch).

Refactor:
 * spec_helper to require git_statistics since it will include CLI class
   and by default it will also include the initialize file
 * The way we pass the branch into the collection method (pass it within
   the options hash)

Add:
 * Default branch constant in CLI class