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.
Like documentation says: "If you do not need the return value to be unique and monotonically increasing, use os:timestamp/0 instead to avoid some overhead."
http://www.erlang.org/doc/man/erlang.html#now-0