Fix for relative datafile paths

Example failure case where image.nhdr contains:

data file: image.raw

>>> nrrd.read('dira/dirb/image.nhdr')
...
IOError: [Errno 2] No such file or directory: 'dira/dirbw'
Add doctest support to nrrd.py when run standalone
More robust header parsing

In particular, fixes parsing headers where the value of a key-value pair
contains a ": " string.

Exposes read_header. Adds documentation and simple doctests. Uses
terminology consistent with the NRRD specification (e.g. field/desc and
key/value).
Merge pull request #3 from nocnokneo/better-header-parsing

Better header parsing