#10 Toxtree integration

[Nina Jeliazkova]

Introduce ability to run Toxtree plugins;

most appropriate is to be integrated within descriptors list and make them visible via descriptor calculation.

[Nina Jeliazkova]

Toxtree refactored to use maven build;
Decision tree classes now implement IMolecularDescriptor and can be used within the descriptor calculation framework.
The relevant descriptors.txt configuration updated to include Toxtee modules.