Truly readable model

Here is Zhen's implementation of a ```--truly_readable_model``` initially for gd. It is done by keeping a dictionary mapping feature names to indexes which is expensive when enabled but does not seem to add any penalty when not used. This feature is often requested and may be handy for debugging small to moderate number of weights. Although it can be done outside vw, this implementation may save one additional pass over the data and handles better quadratic interactions than a 'megaexample' + audit.