Fix the DateTime logic in test.

- This test started to fail on some systems with daylight saving time change.
Switch from facets to aggregations. Closes #144
Add Travis build status image to main README.md
Update URL links to Elasticsearch documentation (from 0.90 to 1.3).
Remove JavaDoc links to Elasticsearch documentation.
Mark 2.0.0 release as SNAPSHOT in RELEASE_NOTES.

- Point Travis build image to Travis Searchisko repository dashboard
#184 - Extend field level security by _source filtering security - basic
implementation and doc
#184 - unit tests added
#184 - _source filtering - functional tests done