Missing an automated test for onDrag

[motiz88]

In the course of working on #6, I have tried to implement tests for onDrag, both for mouse events and for touch events - after all, such tests could potentially have uncovered the original issue (#5) long ago.

Unfortunately React.TestUtils.Simulate.mouseMove / .touchMove did not seem to do the job, so I've submitted #6 without a test for onDrag, but thought I might as well document this omission here for someone else to pick up.