gftools-add-font.py: Needs documentation and usability improvements

Myself and others are having trouble using `add-font.py`, it seems like this tool could use some refactoring and documentation. Marc has said the following to me regarding this tools:

> frankly that should be sorted out before py3
> and it should be done with the GF eng team on board

## Issue One
One issue I run into is needing gflags:


I have found that the python version works, `pip2 install python-gflags`

## Issue Two
I believe some of the paths are relative, this causes problems because the tool must be run from a specific directory, see below: