Formalize IValidator by implementing it (zope.interface optional)

[cdunklau]

As discussed with @Julian, IValidator should be defined formally so we can write tests against things. I'm sure there's other useful things about having it formalized too.

Since it seems silly to require zope.interface for one little interface, the implementation should only be defined if z.i is importable.

Tests against the existing IValidator implementors should be created too.

I'm gonna take a crack at this, let me know if you have input. CC @gazpachoking

[cdunklau]

Ok, the base is done as of cdunklau@7d75408

Still need to clean up docstrings, and understand zope.interface more to ensure I'm doing this right.

Please review and comment as necessary.

[cdunklau]

cdunklau@cb0b2f0 minor tweak

[Julian]

Hey, so appreciated in case I forgot to say that already.

It turns out that z.i can't actually do what we need, as I mentioned this morning (yay!), so this will have to wait until after #99 I think.

[Julian]

This should be way easier to do now if you're still interested, #99 has landed (you can call directlyProvides right inside __init__ of the validator we return I think).