Add timers to core

[brson]

Timers are provided by the runtime, specifically by uv. We will want a core::unstable::timer (to be promoted to core::timer) module with an API that can be backed either by libuv or by blocking implementations.

[recrack]

I think it change from std::timer to core::timer,
not core::unstable::timer
Am i right?

[bblum]

All the above issues are ones I think will be deprecated when the old stuff in libextra is replaced.

[olsonjeffery]

I'm taking this