Add cobertura/coveralls plugins (code coverage).
Enable code coverage reports for travis.
Show code coverage status in README.
Added a basic test for the AuthenticationProvider

Testing if the current version of the Shogun2AuthenticationProvider is assigning the ROLE_USER when a user logs in.
This test has to be adapted if the authentication process changes.
Merge pull request #6 from marcjansen/code-coverage

Enable continuous code coverage reporting
Merge pull request #7 from buehner/security-test

Added a basic test for the AuthenticationProvider