Add coveralls.io support

Coveralls.io can display the coverage of the unit tests.

To generate coverage metrics on Travis, we run the tests with the coverage
package. After a successful build the metrics are pushed to
coveralls.io.

As we supply an empty implementation for a python-3-only function, we don't want
to track the coverage of that code path, as its execution depends on the Python version.
Add coveralls.io badge

The badge displays the current coverage of the master branch.
Add pypip.in badges for current version, license and downloads on pypi

pypip.in provides badges for different metrics pulled from pypi, e.g. version,
license, recent downloads, if the project provides eggs or wheels, etc. Those are
used in the README, to provide useful informations to a visitor of the projects
github page.

The build status badge from Travis is also added to the top of the README, so it
is better visible on the github page.

Other badges should also be added at the top for visibility.
Merge pull request #30 from Moredread/moredread/features/badges

Add pypip.in badges for current version, license and downloads on pypi
Merge pull request #29 from Moredread/moredread/feature/coveralls