Check if device is rooted at all

[J05HI]

Check at the intro if su binary exists. If not show it to the user and maybe redirect to a rooting guide.

[jcgruenhage]

Redirecting them to a rooting guide is out of scope, since that is usually a per device thing, adding a su check when requesting root failed is a good idea though.

[J05HI]

Okay then only the su check.
I've added this to the superuserlib repo.

See playmusicexporter/superuserlib#1