Include the other JNI-generated headers, and fix some detected function prototype problems.
Update documentation for msig_update() with regards to timetag value.
JNI: Add an initializer for TimeTag supporting sec/frac pair.
JNI: Add optional timetag support to Signal.update().
JNI: Add support for msig_value().

User must call Signal.value() with a valid array of the correct size,
and optionally a valid TimeTag object.  The function returns a boolean
indicating whether the signal has a value.