Possible CMake improvements

[traversaro]

The current structure of the library has several CMake-related things that could be improved:

• The version of the library is not explicitly exported
• The exported target does not include a namespace
• Target definition is duplicated, once in the source code and one in the CommonClassesConfig.cmake file.

I would suggest to take inspiration from https://github.com/robotology/how-to-export-cpp-library for the details on how to export a C++ library.

[drdanz]

I strongly recommend to make ycm a hard dependency of this repository.