[Decision Trees] Minor fix to protobuf IDL

Summary: Missed this in a previous rebase

Test Plan: unit tests

Differential Revision: http://phabricator.tullo.ch/D26
[Decision Trees] [UI] Initial UI for decision tree training and visualization

Summary: Basic visualization with d3.js and Angular

Test Plan: unit tests

Differential Revision: http://phabricator.tullo.ch/D24
[Decision Trees] Use dedicated collection for mongo_interface tests.

Test Plan: unit tests

Differential Revision: http://phabricator.tullo.ch/D25