Do not de/index None values, and other index stuff

- it the value to index/deindex is None, do nothing
- calls to the index/deindex methods are done only if field is indexable
- replace "if self.indexable" by "assert self.indexable" in these methods
- hset and hincrby use hget and not hgetall to know value to deindex
Merge pull request #76 from twidi/no-deindex-index-none-values

Do not de/index None values, and other index stuff