Fixed Mobile checking function (isTablet checking was not robust enough for me)

I am not completely sure that what I am suggesting is what was intended by the author, but still.

In the original code testing for `isTablet` was based on the width of the window – and in fact in my case I got it true even on the desktop, where I was using half of my screen width for the browser :)  (Somehow it worked ok on hires iPad, to my surprise).

I suggest to check for `isMobile` as
```
return (/iPhone|iPod|iPad|Android|BlackBerry/).test(navigator.userAgent);
```
instead of based on the window width. (`isTablet` is left in the code, but it is not really used at the moment)