Added option for additional ranking analysis.  

This adds option to add additional trackers to ranking analysis where ranks are computed normaly and additional trackers are then assigned ranks corresponding to their accuracy and robustness performance compared to normal trackers.

When dealing with tracker identifier understrike ("_") will create subscript for following characker in graph legend, which may look silly for e.g. xxxx_2013. I suggest using "-".  

Also in estimate_accuracy.m and estimate_failures.m there seems to be different format for results from year 2013. Instead of having [2] for failure, they have [NaN, NaN, NaN, -2].
Hm ... maybe I see your point, however, I would do it differently. I would leave the default `set_trackers_visual_identity` call in `create_trackers`, then merge the two sets, run `set_trackers_visual_identity` on that and split the set again. It does sound a bit inconvenient, but I think that there is no harm in performing this step multiple times as it comes in really handy for simple analysis and it does not complicate the core functionality.