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
You've added a check for byte[] above, which is good, but the bad cast of Byte[] -> byte[] is still here. While the Byte[] check could be left in, I'm not sure if it offers any value. There's virtually no reason to use a Byte object array over a byte primitive array.