Add bower component paths to sass path

Add components to sass path such that each component can be referenced
with a sass import with just its name instead of the full path.
Simplify adding bower components to sass path

Make use of .bower.json file for each component.  This file specifies
the main directory, which is the file we will want to include.  Use this
path rather than trying to guess the path based on the component name
(this guesswork approach is what we were doing before. it worked but it
may not work for non uniform components)