[Decision Trees] Added Least Absolute Deviation loss function

Summary:
From the original paper at
http://www-stat.stanford.edu/~jhf/ftp/trebst.pdf

Test Plan: Still need to write unit tests

Differential Revision: http://phabricator.tullo.ch/D5
[Decision Trees] Added Huber loss function implementation

Summary:
A fairly vanilla implementation from http://www-stat.stanford.edu/~jhf/ftp/trebst.pdf.

There's a fair amount of jankiness due to the need to pass global information around
to the loss function. An alternative may be to pass the examples directly in the constructor
of the loss function, and recompute deltaM in that form.

Test Plan: no unit tests yet

Differential Revision: http://phabricator.tullo.ch/D8
[Decision Trees] New ArcConfig

Summary: Some minor workflow changes

Test Plan: `arc diff`

Differential Revision: http://phabricator.tullo.ch/D7