Added 'objectOf' PropType validator to iterate on objects and validate properties.

The rationale behind this is when you have object literals used as dictionaries and you want to validate each property (without knowing its name ahead of time). PropTypes.shape allows to validate specific properties, but this rather iterates on the properties of an object (no matter their name) and validates their content (which then can be any of the PropTypes). It works very similarly to arrayOf, so I strongly inspired myself from its code both for the implementation and the tests. 

If you're interested in merging this, I imagine the name can change but I had trouble coming up with something as straightforward and unambiguous.