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/angular2.ts

@@ -1,4 +1,4 @@
 export * from './core';
 export * from './profile';
 export * from './lifecycle_hooks';
-export * from './bootstrap';
+export * from './bootstrap';