Add Indexing #18

It would be useful if SimpleNoSQL could have some sort of indexing on fields to make retrieval faster than the current query mechanism over large data sets. Currently we must pull in all results into memory and apply a filter to remove invalid items. Instead it would be nice if we could apply an @indexed annotation on properties of our models and have them added to a text index for faster search.

This would have to include some sort of mechanism in our queries to use the index for retrieval as well. The regular filtering mechanism should still be available.