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.
Please try to keep the first line of the docs short since they are used as summaries. This should be fine:

```
Creates a stream that emits a value every `n` milliseconds.

The values emitted are an increasing counter starting at `0`.
```

Same for `timer`.