Some constraints among others enforced by this library:
`ButtonGroup`: can have only `Button` children (in fact, at least two)
`Col`: props must be integers between 1 and 12
`Glyphicon`: can't have children
`Nav`: bsStyle must be one of `pills`, `tabs`
For now the constraints are quite relaxed in order to allow an easier demo ({strict: false}), however they can be tighten up to the point of accepting only whitelisted props.
