Fix navbar

Vincent, so sorry for the long delay in getting back to you on this. I've been writing a ton of C++ networking code and it's very hard for me to multitask with that kind of code.

Couple things to keep in mind about this pull request.

* If input$tool is on "A" and you click "A" again in the UI, nothing will be sent to the server. (This is what my {link: foo, unique: i++} structure was supposed to work around, but for your app it didn't seem like this was necessary)
* When the app first loads, input$tool will be NULL. This is currently causing all sorts of warnings. You can either solve this by providing a default value on the <div class="navbar"...> or wrapping your accesses of input$tool on the server in a reactive function that checks for NULL and returns a default value. If you want to set a default, do something like <div class="navbar" data-navbarBinding-lastClick="singleMean"> (if you think data-navbarBinding-lastClick is too much of a mouthful [and it is] then change navbarBinding-lastClick in navbarBinding.js to something else).