Make several hardcoded settings user configurable

This pull-request centralizes all settings / timeout values into the new `riak_ensemble_config` module as well as makes many of these settings user configurable via `app.config`. It's unclear which (if any) of these settings should be cuttlefish'd (nor what the appropriate user-facing names should be).

Addresses #18 

Ah right, I was thinking the function was evaluated at compile time, but you're definitely right. Carry on.