Add draft for requirement combinators and stuff
Add common base for all validators

This removes the need in wrapping all reqs in combinators;
they all behave in the same way so one can use combinators only when needed.
Pretty-print nested exceptions in validators
Support defaults in all reqs
Update year in license note
Forgot the file
Fix test under Py3
Use custom exceptions in combinators. Refactor.

Also commented out the half-assed optimization which requires extra thought.
All syntax sugar: implicit combinators via & and |
Hide a meth from public API
Use a combinator in req.implies
Port validate_dict to new-style validators. And so on :-)
Add docstrings.  Add extra sanity checks.
Add tests for new validators; fix their repr
Add tests for edge cases, fix Py2.x
Use named string format fields (fix Py26)
Merge branch 'master' into requirement-combinators
Update changelog and bump version (devel)
Add translate() func + tests
Add InRange validator, reuse it in Length