Added auto-build hook

Added the auto-build hook which is a bit of a beast. It lacks
documentation at the moment so here is a git config reminder with
defaults:

[auto-build]
	location = /tmp/build/
	max-build = 4
[auto-build "refs/...[*]"]
	destination = /tmp
	backup = yes
	profile = <auto detected>
	command = <from profile>
	pattern = <from profile>