Windows DLL Support

Added support for building protobuf as a DLL.

There is one wonky thing with this, which is that I end up 'installing' two copies of the windows DLLs. One in the lib dir, for other projects to use, and one in the bin dir, so that it sits next to protoc.exe, and that exe is usable. A better alternative would probably be to staticly link protoc, even when protobuf is going to be used as a dll.