Make Sigma a library, would close #85

Changes up CMakeLists to add build options that can build Sigma as Shared or Static library
Also retains the option to build an executable (which links against libSigma)
moves main.cpp to a "src/tests" dir, useful until main.cpp is removed or re-purposed.
Also adds a preference to build with static GLEW, which I like do, but the default is build with shared GLEW (if needed at all of course)