Pull request #90: [java.lang.ClassCastException] When adding HTMLSignature to userConfiguration

Details:
* replaced simple casts to instance of checks
* removed unreachable code
* removed handling for unsigned values because in Java, all types are signed
* replaced EwsUtilities.EwsAssert(...) with Exceptions. I think this Method should also be refactored because checking preconditions with java asserts isn't quite a good choice

* imports optimized
Thanks for reminding me. As you said I also think there should be some strong refactoring on this class. Casting things to Object and back really seems not a good choice.