Don't call toString() on React component children

Don't call `toString()` on children of the <Td> if they're React
components

To check if a variable is a react component this just tests if
`typeof(thing.props) !== 'undefined'`, which is hacky and not very
robust, and a serious case for improvement.

Fixes #76
Merge pull request #77 from glittershark/76/custom-html-inside-td

Don't call toString() on React component children