Add support for making a .deb with CMake's CPack

[joelanders]

Since we're using CMake to build the thing, it seems to make sense to use CPack to package it. To get a minimal .deb out doesn't require adding very many commands to the CMakeLists.txt. Generating a .deb is then as simple as cmake . && make package.

Todo:

• get it to pick up the correct version
• figure out whether we want to install the .so or the .a (both are being built, but it's just installing the .a now).
• figure out a decent default config? or leave it unconfigured by default?