Move to elastic version 1.0

I made the xtas.tasks.es module compatible with elasticsearch 1.0. It
is probably no longer compatible with earlier versions.

The testing for exact result representation bothered me so I also added methods
to get results from elastic search and used those for testing. I also makes sense
to have set/get pairs.

- Updates es module to use _source rather than field in the search.
- Added get_single_result and get_all_results functions to fetch existing
  results.
- Renamed test_backend to test_es and updated tests to check input-output
  consistency rather than a specific representation.