file renaming that follows forthcoming bootstrap convention

hi mattia,

according to the v3.0 thread on bootstrap repo (https://github.com/twitter/bootstrap/pull/6342), with the new promising release every bootstrap variables and filename will strictly stick to the hyphens naming convention instead of camelCase mixed dashes.
now, bootstrap-switch follows this convention as well, for the sake of having an unique component design style perfectly declined on different third-party components.

let me know if you need some changes.