add facet support (taxo)

* support for multi-field facets using lucene's org.apache.lucene.facet.taxonomy
* on index time different analyzers can be used for the facet itself
  for example:
    :documents [{:name "jack doe"}],
    :facets {:name {:use-analyzer "custom-facet-analyzer"}}
    :analyzers {:name {:type "standard"} :custom-facet-analyzer {:type "keyword"}}

* on query size-per-facet-field is also supported:
  {:facets {:name {:size 10} :other {:size 20}}}

* at the moment the DirectoryTaxonomyReader is not reused, new instance is opened
  on every search request