Add Stream.interval/1 and Stream.timer/1

Per discussion on https://groups.google.com/forum/#!topic/elixir-lang-core/lhvytBduo3Q

Tests here are interesting. Anything time based is always a pain to test. Open to suggestions.
It sounds like either we risk the occasional false positive in the test suite or we have a bit more overhead in running the tests. I'm going to leave it as is, since false positives are pretty disruptive.