Revamp and fix PropTypes

This uses the return value (a warning string or nothing) to indicate whether a prop passes validation or not. It used to be done through calling `console.warn` as a side-effect, except this didn't work well with and `oneOfType`, which calls each validator.

The solution was to insert a `.weak` prop to each validator to suppress the warning message. This is overkill and also doesn't work well in because it increases the potential API surface.

So `.weak` is no longer needed: fixes #863.

Backward compatibility for custom validators: since they didn't return anything and directly call `console.warn` or something, this doesn't break them.

Improvements:

  - `arrayOf`, `oneOfType` and `oneOf` got better message now, since they can get hold of their validator's message and output that instead of a generic `warning: bla`.
  - More complete tests, including testing custom types and `isRequired` on everything.

Bug fixes:

  - oneOfType(...).isRequired didn't work. The workaround was to use `oneOfType(a.isRequired, b.isRequired, ...)`. This means `oneOfType(a.isRequired, b)` doesn't make sense. The new version simply makes `oneOfType(...).isRequired` possible.
  - `oneOf([true])` worked for 'true' boolean because it converted everything to string before comparing. It no longer does.