* update .gitignore
* foundation for 3D descriptors
move PBF into core
* cleanup work
* a bit more cleanup
* move the principal moments calc to MolTransforms
* cleanup
* cleanups
* add caching of the principal moments and values
* do not include the 3D descriptors in MolDescriptors.h
* the properties are computed
* add PMI descriptors and tests
* add tests for NPR descriptors
* return 0 when the largest PMI is zero
* PMI edge case tests
* NPR edge case tests
* PBF edge case tests
* PBF edge case tests
* more edge cases
* add a few more 3d descriptors
* add defns to docs
* tests for the new descriptors
* add versions to new descriptors
* add 3d descriptors to python wrapper
* add eigen support to the travis build
* try to get non-windows builds working
* remove computeCovarianceMatrix() from java wrapper
* make pmi property names "private"
