Filter support for polymorphic resources

[ecgunendar]

This is filter support for Polymorphic resource.
Following is the example when this fix works.
need support for oid and type filtering for below example.
Eg ::

class aResource(MongoEngineResource):
  ......
class bResource(MongoEngineResource):
  .......

class cResource(MongoEngineResource):
  class Meta:
    object_class = c
    polymorphic = {
        'c' : 'self',
        'a' : aResource,
        'b' : bResource,
    }
    filtering = {
         'oid' : ALL,
         'type' : ALL,
         'status' : ALL_WITH_RELATIONS,
    }

class dResource(MongoEngineResource):
  class Meta:
    queryset = d
  polyresouce = tastypie_mongoengine.fields.EmbeddedListField(of='cResourcee', attribute='polyresource', full=True, null=True)

[mitar]

We don't import classes directly, only modules.

[mitar]

Please make tests as well.

[mitar]

I am not sure what exactly is this pulll request trying to solve, but please make tests which fail before it and succeed after the changes.