feat(pipes): add support for pure pipes

By default, pipes are pure. This means that an instance of a pipe will be reused and the pipe will be called only when its arguments change.

BREAKING CHANGE

Before:

@Pipe({name: 'date'}) class DatePipe {} defines an impure pipe.

After:

@Pipe({name: 'date'}) class DatePipe {} defines a pure pipe.
@Pipe({name: 'date', pure: false}) class DatePipe {} defines an impure pipe.

Closes #3966

modules/angular2/src/core/metadata.ts

@@ -398,10 +398,8 @@ export interface QueryFactory {
  * ```
  */
 export interface PipeFactory {
-  (obj: {name: string}): any;
-  new (obj: {
-    name: string,
-  }): any;
+  (obj: {name: string, pure?: boolean}): any;
+  new (obj: {name: string, pure?: boolean}): any;
 }
 
 /**