Auto detect timefield

[rashidkpc]

Replaces #169

Phase 1
Timelion's .es() should be capable of matching the index parameter to a Kibana index pattern and should be able to extract the timefield from it, negating the requirement of passing the timefield parameter in the case that the user already has an index pattern.

Phase 2
The .es() function should be able to check the index's mapping for a date field. If there is only 1 it should use it as the timefield in the case that the user did not pass a timefield.