feat($interval): add a service wrapping setInterval

The $interval service simplifies creating and testing recurring tasks.
This service does not increment $browser's outstanding request count,
which means that scenario tests and Protractor tests will not timeout
when a site uses a polling function registered by $interval. Provides
a workaround for #2402.

For unit tests, repeated tasks can be controlled using ngMock$interval's
tick(), tickNext(), and tickAll() functions.

angularFiles.js

@@ -20,6 +20,7 @@ angularFiles = {
     'src/ng/http.js',
     'src/ng/httpBackend.js',
     'src/ng/interpolate.js',
+    'src/ng/interval.js',
     'src/ng/locale.js',
     'src/ng/location.js',
     'src/ng/log.js',