Debian packaging

This set of patches adds a very basic debian package definition.  The package installs to `/usr` rather than `/usr/local`, and puts the documentation in the right place.

There are a couple of minor tweaks to the Makefile necessary to get this to be smooth: firstly, the debian helper scripts expect the Makefile to respect the DESTDIR environment variable, which I've added.  Secondly, the helper scripts install the docs themselves, and don't rely on the `make install` task to do it for them.  This means that before I split the `make install` task out to `make install_files`, I was getting two sets of documents installed, which was a little untidy.

The package installs and works on Debian Wheezy.  I haven't tried it on an Ubuntu, but I think it should work without modification.