Merge pull request #905 from Cadair/image_alignment

merge master into Image alignment
extra coalignment functionality
more capability to the coalignment
more capability to the coalignment: fixed using apply_displacements
filling in co-alignment test suite
filling in co-alignment test suite
fixed test_coalign, and undid changes in the coalignment method brought about by bad test
added in more test functions for the coalignment
added in a small note
tests reveal a mistake in clip_edges! fixed, and implemented test
fixed the mapcube so it accepts pixel shifts the coalignment spits out; also, added a new test function.
finished first draft of test suite for image coalignment
fixes to mapcube coalignment keywords and fiunctionality
removed print statements from test of coalignment functions
tabs replaced with spaces in coalignment testing
cleaned up some extra white space in mapcube coalignment method
added the installation of scikit-image to travis, as per cadair's suggestion, to test image coalignment functionality
trying to install cython
using pip for cython, moved commands as per cadair's request
fixed test of coalignment method