Moves specific list styles from base/defaults into modules/lists

Because the defaults are declared on the `<ul>` and `<ol>` tags, anything qualifying those either need to also be tag-qualified, declared inside this default file, or use `!important`. Those aren't great options. Moving these out into a modules file allows these to be extended more easily and work together on a single element without the ones declared in defaults *always* overpowering the others.
Renaming plain class to list-plain.  Adding documentation to the list module.  Fixing left padding default for lists.
Merge pull request #73 from isitedesign/feature/list-modules

Moves specific list styles from base/defaults into modules/lists