Added new FileField with GridFS support

The API is similar to that of PyMongo and most of the same operations are
possible.

The FileField can be written too with put(), write() or by using the assignment
operator. All three cases are demonstrated in the tests. Metadata can be added
to a FileField by assigning keyword arguments when using put() or new_file().

docs/apireference.rst

@@ -64,3 +64,5 @@ Fields
 .. autoclass:: mongoengine.ReferenceField
 
 .. autoclass:: mongoengine.GenericReferenceField
+
+.. autoclass:: mongoengine.FileField