Avoid click interception

JSTree shouldn't intercept click event.  

Many users tried to get clickable nodes, but click events are intercepted. We can "easily" get around this problem by listening `changed.jstree` event, as mention in the doc : 
> Clicking on the link however will not direct the user to a new page, to do that - intercept the changed.jstree event and act accordingly.

I think this feature request is too recurrent and should be native.

Plus, I came across a deepest problem : what if you've got **others buttons** in your node ?  
See this picture/code for an example : 
![2014-06-13_1736](https://cloud.githubusercontent.com/assets/3398490/3271852/9ced07f2-f311-11e3-876b-761bd81dd666.png)

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              <li>Hello World
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It starts to be a real headache, so why not simply let this click event alone ?

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This PR may be refused for a good reason.  
I tried to search around for it, but I didn't find this reason.