Added documentation to file _main.scss, line 60

Helium.css was setting `vertical-align: middle` to all images,
videos, canvas and audio file. This is so the gap between them
and their containers are removed. Added reference.
Added more consistent whitespace :)
Added animation mixin, simplified arrow docs
Added comment to mark of @if statement
More robust grid system
Removed stripped tables class

The reason to this removal is that stipped tables
is a more of a design-y thing so it wouldn't be a
good decision to keep it in helium.

Bordered tables is pretty much common so not a
problem keeping it
Moved to BEM syntax for naming classes
Added comment to mark end of @if statement
Fixed some class naming stuff