CMake shared libraries

Hi,

since I missed to compile OpenEXR Dlls using the CMake build system, I implemented some CMake options. Moreover I made the usage of the zlib-winapi version optional (via option), it works fine for me using the normal zlib version. 
I needed to change a few files, fixing some export symbols, but finally it works. I tested it on windows 32+64bit Visual Studio 2012.

Since your CMake infrastructure looks quite stable, I strongly suggest removing all Makefiles and VS solutions from the repository.

Best,
Daniel Kaneider