Add webhook support to API sources

[amercader]

While third-party integrations could periodically poll /api/source/xxx/job/yyy (or /api/job/yyyy, whatever we go for on #166) to check if a job was finished, we can make their* life easier if we add support for a webhook that gets called when a job is finished.

This should be really easy to implement for us, as we already have signals that get called whenever a job if finished. At that point we should check if the source has a callback option and if so we POST the job object to it

Tasks:

• Accept a callback param on the source creation/update endpoint, and validate that it is a URL
• Signal called at the end of each job, checks if there is a callback, and POSTs a JSON reprensentation of the job (including report)
• Add support for signing the payloads (callback_secret param)

*by their I also mean ours, as we'll be implementing some of them

[pwalsh]

Excellent!