BUG-648: include yangtools.util package

This include yangtools.util, which will hold various utility classes.
First of which is MapAdaptor, needed to solve the 'copying hashmaps'
problem.

Change-Id: Ifdb7709e1b83d07d1e96832954987072f82274ae
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-648: include yangtools.util package"