add .travis.yml file
Merge pull request #51 from Acpharis/travis_ci

Start Continuous Integration testing with Travis-ci.org

Add .travis.yml file for travis-ci.org
Allow "fast" or "best" for --gen3d options.
Add OBMol::AreInSameRing(a, b) method for detecting mutual ring membership.

Use this method in OBBuilder for spiro ring detection rather than
OBMol::Separate() since it's more efficient and less error prone.
(Current method causes crash in some spiro compounds. Fix pending.)