Add value_decorator into SequenceField

Allows post processing of the calculated counter value.

docs/changelog.rst

@@ -4,6 +4,7 @@ Changelog
 
 Changes in 0.8
 ==============
+- Updated SequenceFields to allow post processing of the calculated counter value (MongoEngine/mongoengine#141)
 - Added clean method to documents for pre validation data cleaning (MongoEngine/mongoengine#60)
 - Added support setting for read prefrence at a query level (MongoEngine/mongoengine#157)
 - Added _instance to EmbeddedDocuments pointing to the parent (MongoEngine/mongoengine#139)

docs/upgrade.rst

@@ -57,6 +57,13 @@ you will need to declare :attr:`allow_inheritance` in the meta data like so:
         meta = {'allow_inheritance': True}
 
 
+SequenceFields
+--------------
+
+:class:`~mongoengine.fields.SequenceField`s now inherit from `BaseField` to
+allow flexible storage of the calculated value.  As such MIN and MAX settings
+are no longer handled.
+
 0.6 to 0.7
 ==========