$(3 :: ExpQ) generates (Proxy :: Proxy 3) not "3"

* also remove the instance Fractional Exp, since those operations
  aren't supported for any Nats
* if you don't have -DUSE_TYPE_LITS, the older representation should be
  generated
Merge pull request #12 from aavogt/master

$(3 :: ExpQ) generates (Proxy :: Proxy 3) not "3"