MOcov/MOcov

This adds an option to the API to specify patterns which should not
be included in the coverage report.

Closes #4.

Will exclude matching directories and files from the list.

For files with no executable lines, the coverage report should
have a nominal 100% coverage rate. This is the behaviour of other
coverage packages, such as those for Python.

Previously, files without any executable lines would report a
coverage of "NaN%", due to the division by 0.

Since the environment variable TRAVIS is set to "true" on Shippable,
the command for Travis-CI should only be run if the SHIPPABLE
value is not "true". Otherwise tests are run twice on Shippable.

"Denominator" is the correct English name for the bottom part of
a fraction.