Made coverBoundingBox() 38% faster by not converting hashes to strings.

Represent hashes by using longs and storing the hash length in the lower 4
bits (which enables for lengths upto 16).
Made coverBoundingBox() 26% faster by avoiding building a TreeMap and using an array instead.
Added tests for CoverageLongs.
fromLongToString: Throw exception if parameter has an invalid length in the 4 least significant bits.
Merge branch 'master' into longs
CoverageLongs not exposed to the user.
Merge pull request #14 from niqueco/longs

Faster coverBoundingBox() by using longs as geohashes internally