Unity3D compatibility.

See https://github.com/NLua/NLua/pull/106
This is one of those cases where I miss the C preprocessor!  Something like this in C++ would make it easier on the eyes for sure.
```c++
#define CLS_NOT_COMPLIANT_ATTRIBUTE \
#if !UNITY_3D \
  [CLSCompliantAttribute(false)] \
#endif
```
Thanks for keeping KopiLua working in Unity!