tried to write an arithmetic encoder on bytes; got stuck
Merge branch 'master' into 256ary
WCTBytes and some tests (very slow ATM)
remove O(256^2) _sanity_check method and make tests pass
Merge branch 'master' into 256ary
wip compute_enc_bit_len()
Merge branch 'master' into 256ary

Conflicts:
	ctw.py
reorganize dirs
enc_len for WCTBinary
enc_len_bytes (analogous to enc_len, but for WCTBytes)
remove obsolete file

(Decided the interface makes things too confusing, so I never ported
WCTBinary and WCTBytes to use it.)