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 

These are all defined at _compile_ time, right? Is that intended?