Check if user is admin - bool

There was no "isAdminUser()" function which returned bool. This is
irritiating as there were a loooooooot of places in the code which
checked this itself with `OC_Group::inGroup($uid, 'admin)` - why not
use a function for this?
(Especially if you consider that we might change the group name in the
future, which would lead to problems then)

Additionally, @Raydiation needed such a method for his AppFramework :)