Added Id attribute to the prefix string.

[gabrieljenik]

The prefix string is built so identifies uniquely each saved element This is usefull when having more than 1 element with the same name. (Ex. checkboxes with the same name)

[simsalabim]

Thanks for the contribution! At a quick glance - it also introduces a bunch of unused variables, but I'll try take a closer look at it later.

[gabrieljenik]

About unused variables: They are inside a method, for a more readable code. :)

[simsalabim]

By method extract you left a bunch of variables unused in their original places. For example: https://github.com/gabrieljenik/sisyphus/blob/patch-2/sisyphus.js#L233