feature(ngEnabled): define ngEnabled directive as negation of ngDisabled

Provide a negated form of the `ngDisabled` directive as `ngEnabled` in
the same vein as the `ngHide`/`ngShow` pair. Allow for slightly clearer
templates by avoiding double negatives, as in:

    <input ng-disabled='!(predicate1 && predicate2)'/>
    <input ng-enabled='predicate1 && predicate2'/>

Closes #1252