fix for issue #5

[schnittchen]

these modifications (up to 32b3a46) make the tests pass (except for one, just as before) on ruby 1.8.7 (tested with ree)

[schnittchen]

sorry, forgot that I had only run part of the tests while debugging. With 81257b8, all but two tests pass:

• "Mongoid::Contexts::Mongo :page :paginator :kaminari should have num_pages" fails on 1.8.7 as well as on 1.9.2 (issue #6 i guess)
• "Mongoid::Criterion::Inclusion#where Geo Spacial Complex Where#near returns the documents sorted closest to furthest" fails on 1.8.7 only. But I have never seen this type of query before...

[schnittchen]

About the second failing test mentioned above: I missed that this was exactly the query syntax introduced by the gem. The test fails with

"geo values have to be numbers: { $maxDistance: 20.0, $near: [ 41.23, 2.9 ] }"

because mongodb geo queries are order sensitive in the hash (swapping $maxDistance and $near would make it work!). Though this could be fixed by using BSON::OrderedHash again, that would break other things... Python has the same issues: https://jira.mongodb.org/browse/PYTHON-292

This effectively means the new syntax introduced by the gem is not usable on ruby 1.8, however geoNear seems to work well, so one can use geoNear (if ordering by distance or obtaining the distance is required) and the old way

Bar.where(:location => {"$within" : {"$center" : [center, radius]}})

and its variants otherwise.

[arthurnn]

Do you mind squashing your commits together, and making sure everything still works before I merge it in ?