Refactor position mixin
Refactor position mixin

- Adds support for `calc()` CSS function
- Fixes #455

nagyistoce/sass-bourbon.io,craftsmen/bourbon,greyhwndz/bourbon,zyj1022/bourbon,chriseppstein/bourbon,Jazz-Man/bourbon,setie/bourbon,nickg33/bourbon,mindeavor/bourbon,Jazz-Man/bourbon,jasonwalkow/bourbon,thoughtbot/bourbon,jasonwalkow/bourbon,greyhwndz/bourbon,ismarkunc/bourbon,smithdamen/bourbon,cllns/bourbon,mindeavor/bourbon,setie/bourbon,Nikeshsuwal/bourbon,craftsmen/bourbon,thoughtbot/bourbon,zyj1022/bourbon,chriseppstein/bourbon,smithdamen/bourbon,nickg33/bourbon