[git] Add bash script to init Github pull request

Adds a bash script that will initialise a Github pull request in a
browser on OS X (depends on `open` command).

Taken from this[1] script.

Using this instead of the similar functionality provided by `hub` as
`hub` actually creates the pull request from the CLI whereas I prefer to
raise the pull request in the browser.

Requires no changes to your Git configuration; just run:

`$ git pr` to raise a PR.

[1]:
http://www.devthought.com/code/create-a-github-pull-request-from-the-terminal/