Fix #21 and change the MQL semantics slightly to match the original MQL
Add test for semantics from #21
Merge pull request #40 from barakmich/mql_bug_21

Fix #21 and test the correct semantics
Replace string type with graph.Direction

This conversion is not complete as there are still uses of string
directions via the Direction.String method in leveldb.
Retain correct copyright notice
Merge pull request #41 from kortschak/enumerated

Yep! Replace string type with graph.Direction
Move iterators into separate package

Also reduce API exposure and use standard library more - and fix bugs I
previously introduces in mongo.
Cleaning and TODOs
Remove dead var
Remove redundant loggin functions
Merge pull request #44 from kortschak/iterator

Move iterators into their own package