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any javascript workunits must be objects that implement the following interface
init(): void
step(): void
is_done(): bool
finish(): string
save(): dictionary
resume(dictionary): void

the string returned from finish is used to upload the result back to the 
server.

step should be made to execute in as little time as possible so as not to lock
up the event loop of the browser, which will cause it to crash.

a sample implementation of a workunit can be found in md5.job.js