PL: updated docstring quotes and other recommended changes

Now set to use triple double-quotes and added recommended spaces. Also added a recommended "super()" method on the SiemensWrapper.__init__
RF+TST: updated multiframe error handling and tests

Necessary attributes from a multiframe stack are now checked for and
raise wrapper errors if minimal information is not present. Some
additions to the MultiframeWrapper docs and reworking of the tests, such
that wrapper errors are checked for based on minimal metadata.
RF: slight refactor of wrapper factory function

Just taste, but I like to test for expected None explicitly, and reduce
if .. else as much as possible.
Merge branch 'nolans-pr' into philips-dicom-extension

Quite a few edits to deal with the refactor of the Multiframe tests into
their own class.

* nolans-pr:
  RF: slight refactor of wrapper factory function
  RF+TST: updated multiframe error handling and tests
  PL: updated docstring quotes and other recommended changes

Conflicts:
	nibabel/nicom/dicomwrappers.py
	nibabel/nicom/tests/test_dicomwrappers.py