Load window.timer from and save it to localStorage

It would be better to preserve the timer value between sessions and not to have to set it in the console all over again.
I don't think `window.timer` is really a useful abstraction here, but rather an interface to the browser console (which accidentally shares the name). The only reason I had to rename `timer` to `_timer` is that all the code here is executed in the global context and hence `timer` and `window.timer` is the same thing.

Ideally, all the JavaScript code should be wrapped in the anonymous function call so that none of the local variables unintentionally leak into `window`. Be it the case, `window.timer` and `timer` would be unrelated to each other and we could keep using `timer` in the code to refer to the actual variable.

For example, `window.timer` getter could put some instructions in the console, and you wouldn't want to trigger that from the page's own JS code.

Check it here: http://jsbin.com/qopoxe/2/edit?js,console