Different init

The original supersized library generate 2 divs automatically:
 1. `#supersized-loader`
 2. `#supersized`

These 2 divs are generated even if the library is not called by
explicit `$('selector').supersized()` call. That is an unexpected
behaviour.

This commit movied the append statement into the supersized
plugin. The divs are now appended only if:

 1. `$.supersized` is called explicitly; and
 2. The `#supersized` div is previously not existed

The behaviour is a slightly different from before, but all other
behaviour reserved.