Support for MultipleResourceDefinition for leshan-client

Hi,
i have added MultipleResourceDefinition in addition to SingleResourceDefinition for the Leshan client just for the feature completeness, the usage can be seen in LeshanClientExample, for instance, powerSource Voltage, Avail.Power Sources etc. 
Also corrected the mockito dependency version range, because the mockito 1.10.? introduced an API break (package structure changes).