Restore 'coverage' config file

This is for testing code coverage of the unit tests using
Ned Batchelder's Coverage.py
See http://nedbatchelder.com/code/coverage/

It was added in
39080f727e35b036b1992934df1c444758e9cc7a
and accidentally removed in
d88d5a54b034d149f318100565f55aceb8e503f9
Updated .gitignore file