WIP: CIFTI2 spec reader and writer

- [x] CIFTI2 reader
- [x] CIFTI2 writer
- [x] tests
- [x] integration with nifti2
@matthew-brett - since i initially built this off the gifti classes i followed the standards there. i usually don't declare class variables up front. is this something that nibabel follows or intended to help docs/py3k?