fix($compile): ng-attr to support dash separated attribute names
docs($compile): improve explanation of Attributes.$observe

The current comment of Attributes.$observe doesn't state correctly the behavior when the attribute contains no interpolation. Specifically, it states that the observer function will never be invoked if the attribute contains no interpolation. However, the actual behavior in this case is that the observer will be invoked once during the next digest loop.
Merge branch 'master' of https://github.com/angular/angular.js
componentizd!