Move TypeScript defs to own folder and add tests

[lostfictions]

Here's a PR for testing TS defs, as discussed in #244. It doesn't test them against the existing spec file -- between Karma augmenting the global namespace and the spec importing some Flow files, it ended up being a bit too tricky. Instead, this PR adds a tests.tsx file that simply verifies that the correct props are supported (as well as being optional.)

(When microsoft/TypeScript#15160 lands it'll be possible to enforce that Draggable and DraggableCore have a child as well.)

I've also added invoking tsc to perform the checking as part of the test task in Makefile.

[lostfictions]

(Incidentally, spotted some invalid/deprecated props in the spec while I was looking things over:

• zIndex here is deprecated: https://github.com/mzabriskie/react-draggable/blob/master/specs/draggable.spec.jsx#L118
• style here should be on the child: https://github.com/mzabriskie/react-draggable/blob/master/specs/draggable.spec.jsx#L629

Not sure why these aren't throwing via PropTypes verification but I don't have any more time to investigate this, sorry :( Just thought I'd point them out!)

[STRML]

Thanks. I removed the root index.d.ts and updated the lockfile. I tested a few invalid definitions and it threw as expected. I appreciate it a lot!