Filter queries as multiple "fq" params

Modifies Solr `select/` queries to send multiple `fq` params

This can help caching efficiency
* http://wiki.apache.org/solr/SolrCaching#filterCache
* http://yonik.com/posts/advanced-filter-caching-in-solr/

This is also mentioned in issue #37

Changes:
* added `multiple_tags_allowed` to `LuceneQuery` initialization (name of this parameter can be changed of course)
* set this new parameter to `True` only for `BaseSearch`'s `filter_obj`
* use lists for level 0 of option serialization
* added `doseq=True` to urllib.urlencode()

I only modified LuceneQuery for `"fq"`. Could other query params be used in a similar way?