Calling hmget without names wont return anything
Calling hmset without data will raise at the redis-py level
In HMSET, use the new internal way to rollback index if case of failure
Add the "hgetall" method to RedisModel for InstanceHashFields
Add hkeys, hvals and hlen commands to RedisModel (for InstanceHashFields)
Make the hexists method of HashField works like other fields.

ie not raise if the instance does not exist, but return False.
Add doc about methods added to RedisModel about InstanceHashField
Use new name of InstanceHashField in documentation
Remove support for setex command

(Because expiration is not compatible with indexable fields)
"decry" and "incrby" are not supported by redis-py, so we remove them
Add the new "bitcount" method to StringField
Add missing tests for incrbyfloat and setbit
Make reset/rollback index methods private
Merge branch 'master' into fields-corrections (RIP_cache)

Conflicts:
	limpyd/fields.py
	limpyd/model.py
	tests/fields/instancehash.py
Merge branch 'master' into fields-corrections

Conflicts:
	doc/fields.rst
Merge pull request #73 from twidi/fields-corrections

Fields corrections