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

Mentioned in commit 9e8c5e3a

Status changed to merged

Mentioned in issue #24 (closed)