Add event keyword search.

Added event keyword search.

Syntax:
* new param "keywords" that takes up to 20 keywords (arbitrary limit chosen to limit expensive queries on common words)

http://localhost:8080/api/event?keywords=ferry+building

Details:
* stop word filtering, lucene k-stem filter for stemming, lucene english-possessive filter,
  lucene standard tokenization (removes punctuation, quotes, etc.)
* all the searchable tokens are stored in the event. See field: searchableTokens
* fields that are used for search. Up to 100 words per event are indexed:
  * title
  * cause names
  * location title
  * description - until token limit is hit.
  
Notes: to test keyword search you have to re-create or re-edit all events. mvn clean + run /bootstrap/test_resources as usual.