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].
You can load them with two separate calls `create_trackers('set_1.txt')` and `create_trackers('set_2.txt')`, merge both to get distinct visual identities and then split them again as you know where to split :)