Fixing namespace of Exception class

Autoloading of the exception class doesn’t work right since the
namespace tells the autoloaders that it will live in
Eventio/BBQ/BBQException.php but it actually lives in
Eventio/BBQException.php.

Added two tests that check that the exceptions are called (the tests
failed in my environment due to the exception class not being found,
but passed after fixing the namespace).

Also adding composer.lock and vendor dir to gitignore.