Some updates for PHP SDK testing and bootstrapping

- Moved phpunit tests in dedicated tests sub-directory
 - Added phpunit.xml configuration file
 - Added test bootstrap, init script and config file for setting up environment
 and configuration parameters (consumerKey, consumerSecret)
 - Updated testapp.php to not display warnings if entities or themes were not
 in response payload.
 - Updated unit tests to not echo output during test runs