Add basic support for input group addons

[seburgi]

This adds the ability to add addons to input fields (see: http://getbootstrap.com/components/#input-groups-basic).

This commit adds two props to the Input component: addonBefore and addonAfter.
Both can either be a string or a renderable element (PropType node).

Example:

    let beforeElement = (<span className="glyphicon glyphicon-search"></span>);

    <Input
          {...sharedProps}
          name="amount"
          id="amount"
          label="How much was paid?"
          addonBefore={beforeElement}
          addonAfter="Just a string" />

Caveats:

• When using an element as addon one HAS to provide an Id, or else the getId method in the component mixin will produce an error when JSON stringifying the props (see: getId() function may bomb due to circular reference #33).
• When using addonAfter, addtional styling for the Input component may be necessary to correctly display the error icon.

[twisty]

This should be this.props.type === 'hidden'.

[twisty]

Thanks for this, looks good. Just one thing I noticed (see annotation on line 61). If you can change and resubmit that would be cool. Thanks again, Tim.

[seburgi]

Thanks for the quick feedback! Damn, you are absolutely right about line 61, I already fixed it and squashed the two commits to one again.