Adds ERXPartial.init(). #602

Here we add an init() method which subclasses (that is, partial
entities) can override. We also implement init() in
ERXPartialGenericRecord, and have it call init() on all the partial
entities it knows about. This works exactly the same way as
awakeFromInsertion(), for example.
Deprecates ERXPartial.awakeFromInsertion(). #602
Merge pull request #603 from paulhoadley/erxpartial-init

Adds ERXPartial.init()