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
The dictionary object can be a byte[] which will be encoded into a base64, and not a base64 string directly.  Please clarify the comment to indicate that.