BF: set parrec data type to unsigned integers

Consensus seems to be that REC data is in uint format - see 
https://github.com/nipy/nibabel/issues/275
We should probably add a check like:
```
if bitpix not in (8, 16):
    raise RuntimeError('Only 8- and 16-bit data supported (not %s)'
                       'please report this to the nibabel developers'
                       % bitpix)
```
I think if a user has a different bitpix value, we want to push them to contact us instead of having unknown consequences.