PyNDN: Adding python exception handing inside callback. Fixing nre

Also, this version makes sure that verification always succeeds
model: Data packet's copy constructor had a bug regarding KeyLocator
ndn.cxx: Bug with timeout-policy

There was excessive and incorrect scheduling of the timeout event
PyNDN: A couple of bug fixes in Name and Data classes
PyNDN: Fixing bug in nre (needed an explicit casting name components to str)