* Add common base DjrillException
* Simplify backend by moving logic
to describe errors into base DjrillException
* Add NotSerializableForMandrillError
for JSON serialization errors
* Add some context to exceptions on unserializable
values (addresses #89).
* Document need to format merge data
(into something JSON-serializable).
* Add RemovedInDjrill2 DeprecationWarning.
* Deprecate blanket date/datetime serialization.
