Add prefix querying to Lucene and Elasticsearch backends and regexp querying to Elasticsearch backend.

Used already to list all of the artifacts in an Ivy repository represented in Titan that have names that begin with "comm".  This is being used on a web front end to link a textbox autocompletion mechanism to a specific property on vertices in a Titan graph.

![image](https://f.cloud.github.com/assets/1697736/706282/fa2870fc-dde9-11e2-9061-cbfe0e6c5634.png)