NF: add support for V4 of PARREC format

V4 appears to differ only in having fewer general and image information
fields.
NF: add PARREC V4.1 support with tests

Add PARREC 4.1 support from information by Gregory Lee at
https://github.com/nipy/nibabel/pull/277
Merge pull request #277 from matthew-brett/parrec-v4-support

MRG: add support for V4, 4.1 of PARREC format

Versions 4 and 4.1 seem to differ only in in lacking a few 
fields of the 4.2 PAR header.