Add operator __in to es.filter() for search in list.

[onegreyonewhite]

This can fix #50 and good update for PR #51

[lauxley]

Yes i think it would be a good idea but does it still work with strings ? .filter(X__contains='test') used to return documents with not analyzed fields like 'foo test bar'. If not we might need another lookup, because it wouldn't be backward compatible and i don't see a reason to break this.

[onegreyonewhite]

@lauxley , I do not quite understand you.
In #50, I raised the question of how to work with lists. I think that it is necessary to share the search for relevant lists and string.
This pull request decide whether to #50.
In order to carry out a filtering of the list of line, I'll think separately.
And I use __in, for backward.

[lauxley]

Adding the __in lookup seems like a good idea, i didn't say otherwise so why do you say you don't understand me ? I can't merge the pull request just yet though, i want to check how it behaves with different types and re-check what problem it solves.
And a test would be nice :)

[onegreyonewhite]

@lauxley, my English is not so good as I want it be. That's why I really can't understand the meaning of some phrases. I understand the words separetly, but the whole phrase sometimes seems difficult to get for me. But I have my wife, who helps me sometimes in English:)