added dbusmonitor.py, which used to be in the dbus_vrm repository
Cleaned and improved examples for dbus handling
BREAKING CHANGE: use None for invalid valued and VeDbusService now acts a dictionary

None for invalid values:
To make the interface to other victron dbus-services a lot cleaner, invalid values are now
represented with None. So there is no isValid parameter to set when setting a value, just do
dbusobject.set_value(None). Same for reading data, no longer do dbusoject.isValid(), just
check if dbusobject.get_value() is None.

VeDbusService now acts a dictionary:
To write use myservice['/Power'] = 12 instead of myservice.set_value('/Power', 12)
To read user print(myservice['/Power']) instead of print(myservice.get_value('Power')
added unittesting to vedbus.py