Don't show the current branch by default

Since retrieving the name of the current branch is an expansive
operation, GitGutter doesn't show it by default.

The `show_status` flag now accepts three values :
  - `none`, where no informations are added to the status bar,
  - `default`, GitGutter will show how many lines have been
    inserted and modified, how many regions have been deleted
    and what we are comparing against
  - `all` where it will also show what is the current branch.