add github post-receive hook listener

[lehmannro]

Github can ping arbitrary URLs when new commits are received (push notifications). A module should listen to such pings and invoke git pull as it sees fit.

Gotchas:

• Spam/DoS protection (how can we verify this is a legitimate ping?)
• Merging (this is a non-issue as this is a clean fetch for deployment purposes-only)
• Runtime updates (how can we inform the core it should do a complete/partial rehash?)

[lehmannro]

I have written a draft implementation in aac824a. Propagation still needs to be added.