Filter queries as multiple "fq" params

[redapple]

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?