Merge branch 'opt_test' into 'master'

Make use of test set on compute_perf optional; Update docs

This MR addresses two issues:

* Users which don't have a test set can now also use `bob_compute_perf`. In this case, only reduced number of statistics and plots are printed
* Fixes to the user guide, mainly concerning how to call the scripts since the simplifications with docopt

See merge request !22