Custom event callbacks and static DW1000 object

• Interrupt handling and configuration is removed from Arduino user code and instead callback functions can be provided to react on certain events. At the moment only successfully sent and received messages can be handled via callbacks. Error handling, other device status changes will shortly follow.
• Now there is just one DW1000 object and no construction of (arbitrary many) ones is intended anymore. From a design perspective code gets easier to use this way, because the proper management of multiple fully operational devices could be tough - and is very likely not even an actual use case.