Support all tether tooltip placement options and update docs

Created by: sethjeffery

Currently bootstrap only supports 4 placement options, while tether supports 12.

Can we allow bootstrap's _getAttachment function to fall back to just routing the placement value straight through to tether if the map is not found? In other words, if I supply {placement: 'top right'} tether should happily receive it and not throw an error "You must provide an attachment".

On a side note, the docs need updating as the "auto" placement is no longer supported.