initial Jenkins setup

[Anil Madhavapeddy <avsm2@xxxxxxxxxxxx>]

All, I've setup Jenkins for continuous build.  It currently triggers off 
changesets to the OPAM repository, and then does a matrix build of different 
compiler versions and package combinations for both normal use and Mirage use.

Because it's behind the CL firewall for now, I've set it to log to IRC 
irc.freenode.net on #opam regularly.  You'll need to SSH tunnel to get access 
to the server itself, so please send me an SSH public key if you want access.  
I'll figure out how to push the build logs to a more public server next week, 
and also to hook in more build slaves for *BSD and the Raspberry Pi.  At this 
stage, it's probably only really of interest to Thomas, Jon, Dave and me, but 
it'll become more useful for everyone as things settle down.

It's already shown that we probably need special support for 'opam install *', 
as the issue with doing 'opam install `opam list -short`' is that some packages 
have a compiler-version constraints, but are still listed, which causes the 
whole mass installation attempt to fail.

Since opam-trunk appears to be broken atm, I've temporarily pointed it at the 
Mirage forks of OPAM on github.  Thomas, we should consider setting the Jenkins 
to autobuild from a staging branch and then push to master to avoid such 
breakages more systematically. There's a plugin to do that that's similar to 
the one used on xen-api.

-a