Experimental WIP to move HDS to Mongoid 4

This includes moving to ActiveModel 4 and get us ready to move
dependent applications to Rails 4.

This is a work in progress and some tests are still failing.
More progress towards Mongoid 4 compatibility
We have some fields that we assign but do not declare. By including Mongoid::Attributes::Dynamic, we tell mongoid to allow us to use fields that are not declared on the model.
Alias fields the mongoid way instead of using the alias command.
Update location of BSON.
In the mongoid 3 gem, running insert on a blank array returned a nil. On the mongoid 4 gem, running insert on a blank array causes a error 13066: "Message contains no documents". If we only run insert when bulk is not empty this fixes the issue.
Mongoid 4 does not seem to handle id overrides the same as Mongoid 3 does. setting field :id, as: :id seems to have allowed us to work around this issue as long as we use self['id'] when referencing our internal ID and any other when trying to get the mongo _id.
Database tasks have been moved outside of the rake module.

https://github.com/mongoid/mongoid/commit/4e0d211f73f63b7932f53ab5140a8609256b10f5
include Mongoid::Attributes::Dynamic on additional models which need it.
Merge remote-tracking branch 'rbclark/experimental_mongoid4'

Merging pull request #145
Updating mongoid to version 4.0.0.beta2
Updating the README