Add support for mongoid 4

There is a problem with the user scopes in mongoid 4.0.0.alpha2. 

When trying to start rails or run tests with an app using 4.0.0.alpha2 and canard the following error is thrown.

    Problem:
      Defining a scope of value #<Mongoid::Criteria:0x007fe4a48d3e30> on User is not allowed.
    Summary:
      Scopes in Mongoid must be procs that wrap criteria objects.
    Resolution:
      Change the scope to be a proc wrapped critera.

     Example:
       class Band
         include Mongoid::Document
         scope :inactive, ->{ where(active: false) }
       end

This should fix the issue. If you want to wait until 4.0 is out of beta to merge this, that's fine.