Provide LFE-native versions of core Extempore feartures

[oubiwann]

Any backends that lack some of Extempore's features will need an approximation of those offered by undertone, written in LFE.

Thoughts:

• Add timing server (with metronome API)
• Add utilities for converting between time intervals and beats
• Add apply-after for beats (analog to timer:apply_after for milliseconds)
• Add support for modes / degrees (e.g., I, ii, V, vi, etc.) with all the convenience functions for generating chords, etc.

[oubiwann]

I had thought this would be needed for the new "Bevin" backend (see #80), but simply sending time stamps or relative intervals does that for those tools.