Workaround for arrays within grouped aggregations. Refs #7
Added tests for grouped aggregations. Fixes #7
Make pipeline key $group recursive.

I am sure there is a bug in this, but its passing tests. Need to look
into it more.
Merge pull request #8 from scottymeuk/feature/recursive-group

Make pipeline key $group recursive.
Allow string to be passed into ensureIndex and deleteIndex. Fixes #9
Merge pull request #10 from scottymeuk/feature/9-string-ensureindex

Allow string to be passed into ensureIndex and deleteIndex. Fixes #9