chore(pipes): rename pipe_registry.ts -> pipes.ts

This is its own commit so that tools have an easier time of preserving
history of the file, by keeping the diff between pipes.ts and
pipe_registry.ts minimal.

Also moved pipe_registry_spec.ts

modules/angular2/test/change_detection/pipes/pipes_spec.ts

@@ -11,7 +11,7 @@ import {
   SpyPipeFactory
 } from 'angular2/test_lib';
 
-import {PipeRegistry} from 'angular2/src/change_detection/pipes/pipe_registry';
+import {PipeRegistry} from 'angular2/src/change_detection/pipes/pipes';
 
 export function main() {
   describe("pipe registry", () => {