Pull request for the validation library

I am splitting the large amount of work done in the last week in different pull requests. This one concerns with the validation library. Notice that some tests seem to be missing, but they are actually part of the next pull request.
Unique and primary key validations are implemented, whereas foreign keys-like validations are missing (but under way).A concept of TableSet (set of related tables) will be implemented next.

https://bugs.launchpad.net/oq-nrmllib/+bug/1237809