Along with this change comes with the removal of Converter.prepare and
adding two arguments to Converter.convert, the context and the argument.
I suppose an added benefit is that you don't have to do attribute
access since it's a local variable.
This allows for you to create converters that can have varying
behaviour using the converter's __init__ instead of having to do a
meta-class based approach to get around the fact that __init__ is part
of the interface.
To make up for the lack of __init__, a new method Converter.prepare was
added to do the work that __init__ used to do.
Rapptz
Khazhismel