Adds a SetUniqueTogether method on TableMap.

SetUniqueTogether adds uniqueness constraints across multiple columns
of a table. Much like setting a single column as unique, this
constraint is used *only* during table creation and not during
insert/update. SetUniqueTogether may be called multiple times, which
sets additional constraints.
Remove accidental "t.Error(err)" at the end of the SetUniqueTogether test
Merge branch 'unique-together' of https://github.com/Nitron/gorp into Nitron-unique-together
generalize trace logging mechanism
Add docs about GorpLogger interface