Relaxed comparison of error message so that it works with MongoDB 2.6.
For $near query, moved the $maxDistance field from a sibling of the $near field to a sibling of the $geometry field, as documented at http://docs.mongodb.org/manual/reference/operator/query/near/, and added an assertion to ensure that the field is being used in the query.
Merge pull request #20 from jyemin/mongdb-2.6-testing

MongoDB 2.6 testing