Expose refresh data method

Fixes #63.
This is sketchy... Wouldn't it make it clear to have these two methods?

1. `resetData(props)` — Uses props argument instead of relying on `this.props`. `componentWillReceiveProps` will continue to check for a change in `dataUrl` and call `this.resetData(nextProps)` when needed
2. public `resetData()` — Does `this.resetData(this.props)`