Add support for NotBlank annotation

[NicolaIsotta]

Fix #738

[NicolaIsotta]

Clean diff: deac4d6?w=1

[ggam]

Nice! A more robust way to do it would be to check the exact class name, no? If you choose this approach, please keep in mind that there's a new @notblank constrainr on Bean Validation 2.0 that would also need to he checked.

[stephanrauh]

There are pros for and cons against checking the exact class name. At the time being, I opt for flexibility. Most likely it's useless, but I render it unlikely that there are disadvantages.