Stop writing to install directory during tests

Right now, the `BaseImageTests` generates test files for comparisions (and the saved files for these comparisions) into the installed directory of wcsaxes. This breaks the tests for any system-wide install.

This saves the test images to a secure /tmp directory, which is then deleted at the end of the test. I changed the pytest option for generating reference images to require a directory to place the images.

This passes all the tests here with `open_files=True, remote_data=True` on Python 2.7, 3.3, and 3.4, except for a handful that I don't think are related: `test_ticks_labels` seems to have a different font weight then the reference, and unclosed file with `test_contour_overlay`.