Move form parameter from __init__ to process on processors.

It makes more sense to pass form to the process() method. It allows you to
easily implement formless processing, as happens with google docs callback-type
deferred processing, because you can instantiate the processor without having
to fake a form.
make save_original_file a class method.
Create temporary directory when attributes accessed

This delays the creation of the tmp dir until it is actually needed. It avoids
creating more than one, which happened because we pre-created the temp
directory even before we fully knew how the import would proceed.