Mountable propType fix

Check mountable prop is truthly, this additional check is necessary since `typeof null === 'object'`. react@0.11 will evaluate prop types before setting their value in some cases resulting in a `null` prop even though a default prop is set. This should resolve #180 #175.

I have also done a quick fix about the use of ModalTrigger for the docs, picked up by @jtlapp.

@stevoland we should probably do a patch release with this fix since it seems to be effecting a few people.