Added a basic entity extractor with tests and example

This initial commit only has exact matches, but as per HTBs wishes I have
tried to keep the format open so that we can easily add more match types
later.

I'm sure this code needs some reviewing, I won't take it personal.
Refactored and reindented.
Merge pull request #11 from comperiosearch/feature/EntityExtraction

Added a basic entity extractor with tests and example.