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.