change the mixin prefix from bi-app to bidi, and add prefix to ALL mixins avoiding compass conflicts

probably controversial changes by github.com/LyoNick:
add prefix to ALL mixin to avoid conflict with compass,
change the mixin prefix from bi-app to bidi (more readable - as in bidi-margin)
left the global variables prefix as bi-app.
this way it is easy to distinct variables from mixins.
maybe change variable prefix too ???