First pass at bundling Cap'n Proto when not already installed.

Just wanted to put this up as reference even though we probably don't want to do it this way. I think it would be much better to pass the capnp source after extracting from the .tar.gz as an Extension. Ideally we can do it in a way so that the setup(...) call compiles and installs capnp and pycapnp using the installation location specified by the user.