Fix log levels

@Vagabond wasn't the first one to [complain](https://github.com/rebar/rebar/pull/142#issuecomment-25289218) about rebar's log levels. To fix that, we introduce `rebar -q` and adjust the log levels accordingly.

* ``rebar -q compile`` - only print ERROR messages
* ``rebar compile`` - print ERROR and WARN messages
* ``rebar -v compile`` - print ERROR, WARN, and INFO messages
* ``rebar -vv compile`` - print ERROR, WARN, INFO, and DEBUG messages

cc @Vagaond @jaredmorrow @DeadZen