Add script to update GitHub hooks to point at ours

Summary: Previously, we used HipChat hooks directly on GitHub to send notifications. Now, the preferred method is to point to our khan-webhooks app which gives us the flexibility to do whatever we want with the info from GitHub. I already set up the hooks manually for webapp, khan-exercises, perseus, iOS. This script sets up the hooks consistently for our remaining bazillion repos. The main motivation for this is to be able to easily redirect all the messages to another HipChat room. (Not all repos had hooks before, but having the hook can be helpful and those are so low-traffic that adding the hook shouldn't bother anyone.)

Test Plan:
Ran `python update_github_hooks.py -n` and saw:

```
Khan/khan-mobile
-- Deleting HipChat hook (61546)
-- Creating web hook
Khan/khan-exercises
-- Deleting inactive HipChat hook (58599)
-- Web hook already created
Khan/khan-api
-- Creating web hook
Khan/sandcastle
-- Deleting HipChat hook (351336)
-- Creating web hook
Khan/gae_bingo
-- Creating web hook
Khan/khan-android-tablet
-- Creating web hook
Khan/live-editor
-- Deleting HipChat hook (109893)
-- Creating web hook
...
```

which matches my expectations for what should happen. Didn't test non-dry-run mode yet.

Reviewers: benkomalo, csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D11402