Adds NFData instance to all RDF implementations

One important caveat: PatriciaTreeGraph requires an NFData instance
for the PatriciaTree implementation of `Graph` in the fgl
library. I've emailed the maintainer of fgl with a patch to add this
instance.
Initial Criterion commit

Compares TriplesGraph, MGraph and PatriciaTreeGraph on

1. parsing a string containing turtle triples
2. querying with a subject, predicate and object
3. selecting with three node selectors
Orphan NFData instance for PatriciaTree
added criterion related gitignore entries