fix(modal): add a work-around for transclusion scope

One of the most common complains about the $modal service is that it is using a transclusion scope for its content. While this is how "AngularJS works" and the created scopes layout makes perfect sense, this transclusion scope still seems to be very confusing for many people... So, here is my attempt at fixing it. The crux of the idea is to use a custom transclusion directive (`modalTransclude`). The fix is rather small and should be "safe". Any feedback highly appreciated! 

Fixes #2110
Closes #2134