Do not activate push state links if meta or shift are clicked.

When using the `{{#link}}` handlebars helper, right clicks and clicks using meta/shift keys do not follow traditional browser behavior. I took a look at [how AirBnB's Rendr is handling this](https://github.com/rendrjs/rendr/blob/master/client/app_view.js#L50-L60), and placed similar checks in the `_anchorClicked` view method. 

Unfortunately, my change isn't quite as robust as I would like. Right clicking still triggers `pushState`, and I cant ctrl+click on OSX to utilize the context menu, however it at least gets things to the point where I can open up links in a new window/tab if so desired.