Stop writing to install directory during tests

Also changes the --generate-reference option to require a directory to
save the generated images.
Keep result_dir in image tests if a test fails

Makes/deletes directories for each unit test
Nicer output on failures by throwing an Exception instead of asserting a
string
Rename and documention option for generating images for tests
Handle missing images in image comparision

Gives path for generating image for use in updating the baseline images
Save baseline image used in comparisions for volatile installs

This keeps baseline images when `setup.py test` fails
Fix generating images with setup.py

Requires some ugly monkey patching of AstropyTest
Also fixes typo that prevented distutils from accepting an argument for
generate-images-path
log10(spacing.value)
Merge pull request #109 from keflavich/issue109

Bad interaction between units and tick labels?
Fixed a bug that caused ticks to not appear if they fall exactly on the position of a frame point
Merge pull request #110 from astrofrog/fix-pv-slice-ticks

Tick labels do not appear for some parameters
Merge pull request #105 from sargas/only-use-tmp-dirs

Stop writing to install directory during tests
Fixed bug that meant that calling set_visible on tick labels did not work correctly (it caused invalid bounding boxes to be added to the list of tick label bounding boxes).
Added changelog entry
Merge pull request #111 from astrofrog/fix-issue-111

Axis labels don't appear in some instances
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