refactor(compiler): rename decorator directives into directive

BREAKING CHANGE:
Previously, `Directive` was the abstract base class of several directives.
Now, `Directive` is the former `Decorator`, and `Component` inherits from it.

modules/angular2_material/src/core/theme.js

@@ -1,6 +1,6 @@
-import {Decorator} from 'angular2/angular2';
+import {Directive} from 'angular2/angular2';
 
-@Decorator({
+@Directive({
   selector: '[md-theme]'
 })
 export class MdTheme {