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
Byte[] and byte[] are not interchangeable. This will either miss a byte[] or throw a ClassCastException when attempting to cast a Byte[] to byte[]. This should deal only with primitive byte arrays.