Added x64 build targets for Visual Studio 10, 11 and 12.

I've added build targets for x64. In the process I've consolidated the intermediate build file location into ./obj with sub-directories for each toolset and platform, I've left the location and file names of the resulting .lib files alone.
I have successfully built these changes in VS 10 and 12 but don't have 11 to test.
There are some compiler warnings for the 64bit version which I can address shortly.