Make and test with coverage

[hugovk]

For #11.

Needs Pillow to be built with CFLAGS="-coverage", for example like python-pillow/Pillow#2781.

[wiredfool]

• This needs to have the various other python modules installed to do coverage.

RUN virtualenv -p /usr/bin/python2.7 --system-site-packages /vpy && \
    /vpy/bin/pip install nose cffi olefile nose-cov coverage coveralls cov-core && \
    chown -R pillow:pillow /vpy

• python ./test-installed.py -v --with-coverage doesn't appear to write coverage for the c layer.

[hugovk]

This branch now pip installs coverage modules.

This branch -- https://travis-ci.org/hugovk/docker-images/builds/287937280 -- is the same but has an updated Pillow submodule which includes the make install-coverage command from python-pillow/Pillow#2781.

Here's a build from that: https://travis-ci.org/hugovk/docker-images/builds/287937280

All are passing apart from three:

• arch: FAIL: TestFileEps.test_showpage
• amazon-amd64: /bin/sh: useradd: command not found
• ubuntu-trusty-x86: FAIL: TestFontPcf.test_high_characters

Comparing with a docker-images master build:

• amazon-amd64: fails in the same way

Anyway, checking just a passing ones for now:

• https://travis-ci.org/hugovk/docker-images/jobs/287937281 shows it is running with coverage! It's covering some non-PIL stuff, and this can be restricted.

[wiredfool]

Looks like Amazon has stripped a bunch out of their amazon linux images, and the dependencies for xvfb haven't been updated. I've fixed that with my pr_12 https://github.com/python-pillow/docker-images/tree/pr_12 branch.

[hugovk]

This PR needs updating for pytest too, re: python-pillow/Pillow#2815.

[wiredfool]

Yes, but the things that are holding it up right now are orthogonal to the pytest issue.

[wiredfool]

Pushing the images now.