Switch to using SSH urls

Haven't been able to test because of 404s on the mock-res dependency, so
this probably doesn't work in its current state
Fix syntax error
Handle both https and ssh urls

Extract functions from assertValid so that it can deal with either HTTPS
or SSH urls, depending on the value of RECEIVER_USE_SSH
Fix reference errors

Conflicts:
	receiver.js
fix tests with https/ssh urls

Test https and ssh url parsing once, and don't worry about it when
testing other stuff.
fix tests
Remove ssh-url dependency
Re-add protocol test for http(s) urls
Document `RECEIVER_USE_SSH` in readme
Use strict
Add `gitstatic` bin
don't rely on push.repository.url existing
Merge branch 'use-ssh-url'
Merge branch 'use-strict'
Get site root + build command from build meta file
support repos with submodules
run git commands in the right order
update builder.sh
actually check cname
check for cname?