The hist command needs some adjustments

@theophile.gentilhomme while evaluating the hist command, I realized by default only eval scores are plotted and are asked for. The docs say:

If you want to
  display dev-scores distributions as well, use ``--show-dev`` option.

But this --show-dev option does not exist. (I think it should be on by default too)

Please also see bob.bio.base#112 (closed)

• There is no way to disable the threshold line
• You can provide -c far but no --far-value.
• --line-styles does nothing in the hist command. Should it be there at all?