No phone/tablet prefix, .*wauto, typo in comment, link to tuto

OH HAI,

I've been reading v2.9.2 and here are my observations (not sure if I should open an issue first that would be closed by this PR or the other way round, well nevermind):

- `.phone-hidden` and `.tablet-hidden` should be completely removed from KNACSS imo: `.small-*` and `.tiny-*` prefixes are now used because they're agnostic to physical device widths and they just correspond to LESS variables that can be freely modified.  
**Warning:** there'll be retrocompatibility issues for people still using `.tablet-hidden` and `.phone-hidden` instead of `.small-hidden` and `.phone-hidden` but well, these are the last 2 occurences of these old prefixes (I think we've moved from them circa september 2012)  

- Helper class `.small-wauto` exists in relevant MQ but neither `.tiny-wauto` nor desktop `.wauto`.
  - I can't remember if I've used the former class that much (I think `.tiny-w100` was more useful), but that would be more consistent to have both prefixes. 
  - `.wauto` is useful when a module has a default width but then some occurences shouldn't have one  

- the flexbox part of KNACSS has a [tutorial](http://knacss.com/demos/tutoriel.html#flex) with examples of code. It'd be useful to have a link directly into relevant LESS code