BVAL-337 ConstraintViolation

Feedback welcome @gmorling @hhferentschik

I ended up:

- adding getExecutableReturnValue and getExecutableParameters
- make getRootBean return null for Constructors (it's really a return value)
- still make getRootBeanClass return the class hosting the constructor

The problem is that Constructor is a "static" method if you think about it.