titles for histograms.

option

plots

Before only one title could be changed even when two plots where in the
figure and title was augmented with (development/evaluation). Now the
titles can be entirely defined.

Correct display of available criteria

Modify the display of histograms: now can use subplots to display several histograms on one page, can no longer display both dev and eval histo at the same time, can control the number of columns in the legend

Add min-far-value option, rename -l (already used) to -Z, change the way linestyles are handled and make use of them optional

Replace a force_eval option so that no evaluation option is needed when evaluation scores are mandatory

Change the way the scores arguments are passed to the compute() function: it now does not rely on dev,eval pairs anymore and can take any number of different files (e.g. train)