Add custom Builder section styles to shrink icons a bit

This preps us for adding a couple other areas in the near future.

At some point I want to do a better job of managing the upstream
CSS with our custom CSS and will likely introduce a grunt process
to help take care of this. For now, we'll append to upstream and
just try to keep track
Each section icon can use a third of the area
Rename spine_get_two_column_data() to spine_get_column_data()

Adds an argument for the number of columns so that we can reuse
this for three column layouts
Introduce three column layout, thirds

We should look at how this can be triptych as well.
Provide a layout selector for three columns - thirds or triptych

Sets a framework for doing the same with two column layouts
Add support for side-left, side-right, margin-left, margin-right

These are applied to their respective two column layouts. We've
left halves alone and will need to come up with a longer term
two column strategy where you choose the layout once you choose
two columns. This could involve deprecating the current two
column options.
Introduce a 4 column layout

This is basically a duplication of the three column layout and
really highlights how we should be doing this with a "columns"
layout that allows you to select a number. But oh well. :)
Documentation around spine_output_builder_section_layout
Clean up the display of the advanced controls area a bit
Merge pull request #116 from washingtonstateuniversity/sections

[PR] Additional builder templates and options for sections