Support 2dsphere queries

This PR:
 * Fixes #102
 * Incorporates @patkar's commits from #108 
 * Cross-references doctrine/mongodb-odm#530

Todo items:
 * [ ] Ensure `$maxDistance` works with `$near` 2dsphere queries
 * [ ] Support `$centerSphere` with `$geoWithin`
 * [ ] Tests for all [geospatial operators](http://docs.mongodb.org/manual/reference/operator/query-geospatial/)