Reorganized Unity project to be more "plugin" compliant.

Added .sln for spine-csharp that compiles under MonoDevelop (mac).  
Modified spine-csharp.csproj to compile under MonoDevelop (mac).
Moved spline-unity .cs files into an Assets/Plugins hierarchy for cleanliness/standard compliance and to make interoperation with UnityScript (.js) projects easier.

Someone with Windows should verify the .sln and .csproj still work under Visual Studio.