changed .hms and .dms attributes to output a named tuple instead of a regular tuple
fixed tests to match new dms/hms
fix docstrings to reflect namedtuple dms/hms
added absdms attribute to Angle
Update changelog
Ignore warning only on Python 3.4
This change to itersubclasses restores some determinism to it on Python 3.4.  See https://github.com/astropy/astropy/pull/1945#issuecomment-32522217

Conflicts:
	astropy/io/fits/util.py
Fix importing argparse on Python 2.6
Clarified in docstrings that named tuples were being returned as suggested by @cdeil
Add a negative examples case to the docs as suggested by @cdeil
Simply absolute value-taking for dms as suggested by @mhvk
Changed `absdms` to `signed_dms` as suggested by @mhvk
Fixed bug in Model2DKernel
Added type cast to float64 in CustomKernel and adapted peak normalization
Added simple ModelKernel tests
Clarified MexicanHatKernel normalization
Raising exception when normalizing kernels, that sum to zero.
Removed remaining mentions of `absdms` as pointed out by @cdeil
Adjusted examples to only do negative angles where there's a point (suggested by @mhvk)
changed named tuples for hms/dms to lower-case convention at the suggestion of @astrofrog