Accept Java's inability to represent unsigned quantities and work with it instead of against it.

Keep our "0x" prefix inside the potential sign character and
explicitly remove it before converting the string representation to a
BigInt.
Provide a means to verify the test actually passes.
Merge pull request #349 from ucb-bar/badsign347

Badsign347 - Accept Java's inability to represent unsigned quantities and work with it instead of against it.