added coverage badge to README; coveralls gem

Hi, I've added the coveralls badge to the README:

[![Coverage Status](https://coveralls.io/repos/lemurheavy/client_side_validations/badge.png?branch=3-2-stable)](https://coveralls.io/r/lemurheavy/client_side_validations)

Here's the repo on coveralls:
https://coveralls.io/r/lemurheavy/client_side_validations

After merging, the README badge url should be updated to use the ```bcardarella/client_side_validations``` instead of ```lemurheavy/client_side_validations```

Thanks!