Add srp/3 that returns number of assumptions
Implement srp_4/2 that correctly solves SRP of size 4

Implemented in srp_4.pl.
Uses constraints correctly (hopefully). Reasons to believe that:
* labeling makes more than zero assumptions
* all clpfd constraints are posted directly in body of srp_4/4
Fix srp/2 test kleinberg_2

Now expects to get exactly the two correct solutions with no duplicates.
Fails in current implementation of srp/2.
Complete overhaul: Re-implement srp using scores

Uses constraints properly.
Temporarily "fixme" disable srp test wiki_marriages

This test doesn't work properly in the new implementation of srp.
Need to investigate.
Add NaturalDocs-compatible documentation to some predicates
Delete srp_4.pl (obsolete)