first draft of postings highlighter:
- supports multiple values (with limitation below)
- supports both _source and stored fields

Limitations:
- doesn't check for supported queries
- doesn't support require field match, return snippets with no highlighting if there's no field match (lucene bug?)
- when number_of_fragments=0 we return a single snippet out of multiple values (containing the paragraph delimiter) while other impls return a snippet per value (will try to address that later on)
added comment to CustomPassageFormatter
forked lucene postings highlighter
made sure that we don't return snippets without matches to be highlighted.
Added proper tests for CustomPassageFormatter
added postings highlighter tests taken from lucene to test our fork
added support for require field match
added our own tests that relate to the custom features we built and/or use
clarified comments and moved up extracted method
removed check for empty fragments in custom formatter
we solved that problem overriding getEmptyHighlight, way better
Updated tests and added comments
added support for discrete highlighting and applied LUCENE-4906
fixed method name
changed default max length, added our own per doc and per field api
improved comment
added comments
removed unused cache
added query rewrite static method (unused for now)
no need to check the query
improved snippets sorting algorithm and added new hlType
added integration tests for postings hl
synced with lucene trunk to incorporate LUCENE-5176 fix