helper+docs+examples: New helper to simplify link failing

Thanks to Saran Tarnoi for providing base implementation of the code
model: Fixing warning in ndn-face.cc

Thanks to Saran Tarnoi for reporting the problem
utils: Small memory optimization with trie data structure
docs: Fixing typo

Thanks to Jin Xue for reporting the problem
PyNDN: More fixes
model: fixed warning in ndn-face.cc