Split parsing+formatting to a separate library

Elixir v1.8 will include timezone support, which will allow Timex to shrink a bit. As Elixir calendar support improves, we want to ideally support it by an ecosystem of focused libraries. With this in mind, I would like to propose for timex to be split into:

* Timex "core" - duration, interval, shifting
* Timex parse+format - parsing, formatting (and the tokenization backbone)

This will allow people to cherry pick the features they need and complement with what Elixir core provides.