QueryExecuter: Improve the way we do AND searches

* Use StoqNormalizeString when generating the clauses so we can search ignoring
  accents too.
* Use the "pg_trgm" postgres' extension that will speed up some indexes
* Create indexes for stoq_normalize_string on sellable.description. Together
  with pg_trgm it will be a lot faster to search for products on a database
  with lots of them (tested on a database with 50000 sellables)
* Reduce *a lot* of overhead from stoq_normalize_string. Use a list suggested
  by django that should be more than enough for us.

Change-Id: I90cf6d8b336bcb2dd72cc4264939bb6389290354
Merge "QueryExecuter: Improve the way we do AND searches"