[Issue 97] Add logic to filter by None.

ElasticSearch doesn't really allow filtering by None, instead you have
to make a filter that asks "is this field missing?". Weird. This turns
`key=None` filters into `"missing": {"field": "key", "null_value":
true}` queries. This will match documents that have key=null, or if key
is set to a null-like value (which can be defined in the mapping).

Fixes #97