CMake Cleanup

Super cool that you started this :+1: 
Having just one build system (cmake) for all OS and IDEs is so much nicer than having a autotools, a Windows etc build system.

Restructured the file a bit. I don't know if they would accept the clientversion change upstream, but IMO the version should be defined in just one place, and not have it to change in many places. Either in the build system or even only as git tag like we do it in Open Transactions (https://github.com/Open-Transactions/opentxs/blob/develop/CMakeLists.txt#L9).