Possible improvements in CMake configuration

[firegurafiku]

Hi!

I've looked through the project build configuration, and I found few moments I can improve it:

• Merge all CMakeLists.txt into a the topmost one. You cannot build only part of the project by issuing cmake ../src or cmake ../clewTest, so why have them separated at all?
• Options like BUILD_TESTS or BUILD_SHARED_LIBRARY should have CLEW_ prefix in their name.
• Maybe rename clewTest directory to clew_test or simply test?

What you you think about the proposal?
Option renaming is a breaking change, so I'd want to discuss it prior PR'ing. See the EasyCL issue discussion for more information why this rename maybe a right thing to do.