Enhancement: expose ping() on pingable-connections.

This PR introduces a new PingableConnection interface and exposes
a ping method on \Doctrine\DBAL\Connection. For drivers where no
ping is supported, a \Doctrine\DBAL\ConnectionException is thrown.
Small cleanups, simplification and removed no longer needed cast
Bugfix: coding standard & missing assertion
Enhancement: implement ping() for all drivers

 * h/t @stof and @deeky666
 * add two test cases for the general connection test
 * remove the ConnectionException method I added originally
Forgot removing method from Driver interface, documented ExceptionConverterDriver better.
Fix postgresql test on travis
Enhancement: CS fixes

 * license
 * author tags
 * doc blocks
 * new lines
Enhancement: use connect() to simplify the test
Merge pull request #414 from easybiblabs/topics/DBAL-275

Enhancement: expose ping() on pingable-connections.
Merge pull request #416 from kimhemsoe/mysqli_cleanups

Small cleanups, simplification and removed no longer needed cast
Fix Connection#ping() to have clear behavior, refs GH-414