Add the possibility to have a custom scope

This line adds the possibility to use a custom scope with $ionicLoading 

I found it useful to show the progress of the loading process.

Demo: http://codepen.io/anon/pen/Ltign
Merge pull request #2372 from ankri/patch-1

feat(ionicLoading): Add the option to use a custom scope
fix(ionicons): only apply spin to :before, and not the entire element

Closes #2416
fix(ionOptionButton): vertically align content

Closes #2348
Merge pull request #1 from driftyco/master

Update from origin