Moving pandasjson back into mainline pandas

This extension works perfectly fine on Python 2.x and all platforms to my knowledge except win32 on MinGW (it's fine with VC2008+ AFAIK). It might have problems on some of @yarikoptic's esoteric Debian platforms, we should identify which ones those are and disable the extension in the setup.py until we have a chance to sort out what's wrong in the C code. I don't think that's a bad compromise for now in the interest of having a usable extension for a lot of folks.