Added checks for incorrect usage of innerHTML. Fixes #1370

This PR adds checks to prevent the incorrect setting of inner html, to make it easier for developers to spot hazards in their code and avoid security holes.  There have been several discussions about how best to handle the innerHTML issue ( most notably, the public discussion here: https://github.com/facebook/react/pull/1515 ), and the conclusion of these discussions was the creation of issue #1370, which is fixed by this commit.
Warnings should move into the `__DEV__` block so that they never execute in prod.

We don't do that automatically, partially because we don't have good dead-code elimination but mostly because we don't know if there is extra work that can be removed in production rather than just this statement.