Update astropy_helpers sub-module pointer to v0.4.2
Ensure that we only create the frame path once and then update it, rather than create a new one every time, otherwise matplotlib uses old clip paths when plotting objects.
Added images regression tests for frame clipping
Added changelog entry
Temporarily disable Numpy 1.5/Matplotlib 1.2 tests
Merge pull request #113 from cdeil/ah-update

Update astropy_helpers sub-module pointer to v0.4.2
Merge pull request #117 from astrofrog/fix-frame-path

Patch needs to be updated as soon as it is accessed
Update
Add license line and clean up imports a bit
Merge pull request #114 from cdeil/datasets-update

Datasets update
More subtleties relating to frame paths - if WCS is reset a new frame object is created, but need to preserve path object.
Added changelog entry
Merge pull request #120 from astrofrog/more-frame-fixes

More subtleties relating to frame paths
Added get_axislabel method
Added changelog entry [ci skip]
Merge pull request #122 from astrofrog/get_axislabel

Added get_axislabel method
initial implementaion of displaying world coordinates, also for overlay coords
bound preferences for display coordinates to key events
displayed world coordinates are now formatted
wrapping coordinates for longitude type, now displays world coords for data cubes