admin UI tidy up

I'm sorry, but a lot of the text on the settings page wasn't very clear. The attached patch rewords the UI text and also moves the admin form into its own separate file, drupalgapps.admin.inc, which is a bit more standard. By having it in a separate file, that code is only loaded when it is needed and helps improve the site performance ever so slightly, especially on the modules listing page. I also added doxygen style comments to group functions logically together.

Dependant on #685606: coding style cleanup and #685670: modularlize the code being applied first.

Cheers,
Stella