Highlighting keybindings

A little playful code to show the user when an Arethusa keybinding is invoked.

It's activated through setting in the configuration of `main` the property `"showKeys" : true` and using the directive `keysToScreen` (the `main_with_sidepanel.html` makes use of it already)

The `keyCapture` service is looking worse and worse. It's API is fairly cool - the code itself is a disgrace. Time to refactor when we work on #292.