Added C++ and Python helpers to retrieve force-field parameters

- added C++ and Python helpers to retrieve force-field parameters
  (i.e., equilibrium distances/angles and force constants) from
  UFF and MMFF in response to two requests recently appeared
  on the RDKit-discuss mailing list:
  http://sourceforge.net/p/rdkit/mailman/message/32953737/
  http://sourceforge.net/p/rdkit/mailman/message/32880156/
- did some clean up on the MMFF code
- NB there are two ABI changes:
  1) StretchBendContrib(ForceField *owner,
       const unsigned int idx1, const unsigned int idx2, const unsigned int idx3,
       const MMFFStbn *mmffStbnParams, const MMFFAngle *mmffAngleParams,
       const MMFFBond *mmffBondParams1, const MMFFBond *mmffBondParams2);
     previously was:
     StretchBendContrib(ForceField *owner,
       const unsigned int idx1, const unsigned int idx2, const unsigned int idx3,
       const std::pair<bool, const MMFFStbn *> mmffStbnParams,
       const MMFFAngle *mmffAngleParams, const MMFFBond *mmffBondParams1,
       const MMFFBond *mmffBondParams2);
  2) std::pair<double, double> calcStbnForceConstants(const MMFFStbn *mmffStbnParams);
     previously was:
     std::pair<double, double> calcStbnForceConstants
       (const std::pair<bool, const MMFFStbn *> mmffStbnParams);
  The two changes are NOT mandatory - however, both the StretchBendContrib constructor
  and calcStbnForceConstants(), though public, are basically "internal" method that
  most likely no-one has ever invoked. Given that the current API is MUCH better
  and cleaner, I would really advise for the new version.

- the change in rdkit/Chem/TorsionFingerprints.py is completely unrelated.
  test 82 was broken on my CentOS 6 Python 2.6.6.