Supply CMake module for stm32plus

This PR addresses #24.

We've been using this in-house for the last few months, and it's worked really well for us. Most of the merge is actually the `system` folder which is part of the example. The key files to check out are:

SConstruct
cmake/SConscript  (comment explains how it works)
cmake/stm32plus-config-specific.cmake.in
cmake/stm32plus-config-version.cmake.in
cmake/stm32plus-config.cmake.in
cmake/example/CMakeLists.txt  (to see what the usage is like)