WIP: CIFTI2 spec reader and writer

- [x] CIFTI2 reader
- [x] CIFTI2 writer
- [x] tests
- [x] integration with nifti2
to be clear, we leave the `to_cifti` method in `Nifti2Image` and the new `is_cifti` property?

```
from nibabel.nifti2 import load
img = load('cifti.dtseries.nii')
```

should the above return a Nifti2Image?