Added offset classes.
- .offset# allows span left spacing without adding empty .span#'s

Added break class.
- w/ appended .break class floated spans won't stack inline. Allows span right spacing w/o adding empty .span#'s

(see example.html for implementation.)
Merge pull request #4 from cluesang/master

Added offset classes
Build latest css
Added examples for picture element and srcset
Merge branch 'master' into gh-pages