Parameter constraints

Adds the ability to specify regex constraints for param values on individual routes. Example:

```javascript

var constraints = { id : /^\d+/, action: /^(create|read|update|delete)$/ };
React.renderComponent((
    <Route handler={SiteWrapper}>
        <Route name='posts' handler={PostPage} path='/posts/:id/:action' constraints={constraints} />
        <Route name='404' handler={NotFoundPage} path='*' />
    </Route>
), document.body);

```

In this example, the path `/posts/123/read` will route to `PostPage`, while the paths `/posts/abc/read` and `/posts/123/foobar` will route to `NotFoundPage`.