added image unit tests
forgot to add the test_images file
added nose to travis.yml
fixed code to copy refrence images for python setup.py test
trying to figure out overlay_shapes failure
trying to figure out cause of failure by brute force
hopefully image tests pass now
added more tests
added missing reference images
added pillow to optional dependencies for travis.yml
using pip install for pillow
installing pillow in separate line in travis.yml
Merge pull request #200 from anizami/image-tests

Adding image unit tests
updated aplpy,axis labels, tick labels, grid, colorbar and frame
added slicer, made formatting edits in other files
edited ticks and wcs_util
minor changes of _ax1 to ax
various bug fixes
only one test with formatting scalar coordinates with % fails now
removed contour_util file, also noticed contour test failures