CMake support for CapnProto

For #43

Ported directly from Makefile.am, so mostly in the same order, with
plenty of artifacts.

Things to complete;
 * [x] get capnp_test to link; currently disabled in a `if(0)` block
 * [ ] configure cpack to output the files for a package
 * [ ] probably write a `FindCapnProto` module
 * [ ] along with something similar to Protobufs `PROTOBUF_GENERATE_CPP`
 * [ ] cleanup what remains