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
NF: add guard for unexpected bit widths

From comment by Eric L
Merge pull request #279 from matthew-brett/parrec-as-uint

MRG: PAR / REC data are unsigned 8 or 16 bit integers

Consensus seems to be that REC data is in uint format - see 
#275

Raise error for data width other than 8 or 16 bit because we haven't any examples to know what these are.