Copy openal32.dll alongside the executable

[ChrisThrasher]

Anyone using the Audio module on Windows is going to run into this problem. Because there is no corresponding CMake target for that DLL, CMake can't know to automatically copy it like it does all other DLLs. An upstream fix to SFML/SFML would be preferred but maybe for now we automatically copy that DLL whenever someone is using Windows.

[ChrisThrasher]

We've made some progress in SFML 3 to improve this situation but we really just need to add some code to this project automatically copies the correct DLL for their current architecture. It's a bit tedious but doable. That way users can use the Audio module without issue. I've always wanted to fix that long standing issue. This will be simpler if #19 is merged because then we always know where to copy openal32.dll.