Properly serializing ObjectId fields.

Data drivers now expose a 'serializers' dictionary. This is then used by the
new eve.methods.common.serialize method to properly serialize certain field
types according to specific database needs.

ObjectId fields different from ID_FIELD are now properly serialized on
POST/PATCH/PUT methods, and can be filtered accordingly.

Limitations: eventual objectid and datetime values in nested documents are not
being serialized yet.

Closes 142. Partially addresses #136 (WIP).