Ignoring case on git route regex

Hi @mirskytech,

Thought I'd add a patch for the `arm install git....` [issue](https://github.com/mirskytech/ansible-role-manager/issues/3) I reported recently, I've manually tested it with all of the examples in the [docstring](https://github.com/mirskytech/ansible-role-manager/blob/master/arm/routes/git.py#L8) (though needed to prefix `git+` on one of them, see below) 

Line 21 of http://mirskytech.github.io/ansible-role-manager/_sources/specifiers.txt will need a `git+` prefix to conform.

Ill keep an eye on the project, if you add a test suite ill add an automated test to support this, in the meantime, to test, checkout this branch and:

`arm install git+git://github.com/Ansibles/build-essential`

The problem turned out to be simple, github uses upper and lower case in their user/organisation/repo names :)
`alias_match` was `None` so had no `groupdict()` method, I am returning the original `identifier` instead