SERVER-9395 Implement $minDistance for geoNear command

Squashed:
- Spelling in comment.
- $minDistance option for $near queries with 2dsphere index. SERVER-9395
- Implement $minDistance for geoNear command. SERVER-9395
- uassert that geoNear's min/maxDistance param is non-negative
- More $nearSphere tests. SERVER-9395
- uassert that minDistance isn't used with 2d index. 2dsphere index is required.
- More informative error if assert.throws is misused.

Signed-off-by: Matt Kangas <matt.kangas@10gen.com>