Added coveralls and coverage to TravisCI

Coveralls is a CI for test coverage. Used the instructions on
https://github.com/coagulant/coveralls-python
Restricting coverage report to exclude some files

Test coverage in coveralls was way less (28%) than nosetests reports
(48%). So created the .coveragerc to exclude some dirs from the report.
Added Coveralls badge to README
Added a TraviCI dir to omit in coverage reports

Seems that low coverage reports in Coveralls are because of Python
packages in /usr/share/pyshared/ (where Travis keeps them?).
Added coverage command to Makefile
Ignoring .coverage
Merge branch 'master' into inversion