More CMake Improvements

These changes:
* Split the CMakeLists files for capnp and kj
* Add support for building/linking to a `CAPNP_LITE` version of the library
* Add support for pkg-config
* Fix #136 

The only major feature missing from the CMake build is version information for building shared libraries. The issue I ran into (other than updating the release scripts) is that CMake uses a `<library name>.<version>` filename pattern for library outputs, while automake uses `<library name>-<version>`. This isn't configurable for CMake; I don't know about automake.