Generate coverage data and prepare integration with Coveralls.io

I've updated the Travis build file and the Tox config to run nosetests
with the --with-coverage parameter. The resulting coverage information
can then be automatically submitted to Coveralls.io after each Travis
build (via the coveralls module).

I've also embedded a coverage badge to README.md so that we get a nice
coverage status once the repo is connected to Coveralls, i.e. something like 
this ![coverage](https://coveralls.io/repos/dbader/schedule/badge.png)

This builds and tests successfully on py26, py27, py32, py33, and pypy.

All that's left to do for coverage data to appear is: 
- Connect the `nvie/times` repo to Coveralls via [this link](https://coveralls.io/repos/new)
- Trigger a build