Add box-sizing rule to :before and :after.

As per the discussion in #23.
Update demo stylesheet with 753f322
Merge pull request #32 from hugobessaa/feature/pseudo-box-sizing

Add box-sizing rule to :before and :after
Fix typo
Update readme
Add gulp-header
Bump version
Fix the header new lines
Add a11y gulp task
Change hover color on .btn, closes #25
Fix merge
Update css