BUG fix; Hashing across words; Clearer API;

0. Fixed BUG in _jelly_write when using mask to write entirely to one word
1. Added JH_FORCE_64 and JH_FORCE_32 to force word size
2. Changed API to only use char* instead of word size arrays (HWord now internal only)
3. Added twang 32 bit hash, also used for top word of 64 bit entries
4. Added more rigorous hashing that mixes across word boundaries
5. Renamed marco constants NRBITS -> JH_NRBITS, HASH_NULL -> JHASH_NULL
6. Updated readme