Add support for git repositories in the 'includes' directive.

Now, the 'includes' directive supports the following sentences:

includes[remote][makefile] = 'drupal.make'
includes[remote][download][type] = "git"
includes[remote][download][url] = "git@github.com:organisation/repository.git"
; branch could also be tag, code relays on make_download_git function
includes[remote][download][branch] = "7.x"

* Correcting code style according to Drupal and Drush standards, as pointed by @dickolsson

* Correcting strange indentation after last fix

* Correcting string concatenation as pointed out by @nbouhid
Adding corresponding unish test for git support feature on includes directive

* Moving test method for includes from git instead of having it on a separate class, as pointed by @jhedstrom

* Standardising comments on makefiles based on @dickolsson's feedback.

* Standardising comments as pointed out by @dickolsson
Adding corresponding documentation to new download makefile from git repository feature