Add ordereddict dependency for Python 2.6

For Python2.6, add dependency on the ordereddict module in setup.py,
and report an error in pwb.py and tests.__init__ if the ordereddict
module is not available.

Change-Id: I09388eee1ceccd9d95c34fef4dbd2e3d421c162c
Merge "Add ordereddict dependency for Python 2.6"