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

- 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
Fix #90:[java.lang.ClassCastException] When adding HTMLSignature to userConfiguration

- imports optimized