MAX!Cube binding improvements

This pull requests includes the send command functionality (https://github.com/cyclingengineer/openhab/tree/maxcube-binding) initially developed by Paul (@cyclingengineer) as well as the bugfix for NPE (https://github.com/aheil/openhab/commit/2177eb1f510de5f2c57352ca5ad94f0e01a3720a) due to unsupported devices by  Till (@dereulenspiegel). This pull request also improves also the lifetime of devices managed by the binding, e.g. value changes received from the MAX!Cube lan gateway are now managed in a better way.