Update to Mongoid 3

- Now requires Mongoid 3.0 as minimum version (this is the only version to support Rails 3.2; Mongoid 2 is very outdated and few plugins continue to support it.)
- Upgrade specs/helpers for Mongoid 3
- Support Mongoid aliased field names (e.g. field :foo, as: bar, type: String). Includes spec
- Support before_type_cast on Mongoid. No code change was required for this, only a spec change.
- There was one mongoid spec that was previously marked pending. I've found that it now works on Mongoid 3.
- Removed some Mongoid 2.3 and earlier hacks, which simplifies the Mongoid ORM code.