Fixes #47 - Make content 100% height of available area

Height for content was 100% of window size, making it include the
height of the header, which caused a scrollbar to appear unnecessarily.
This update sets the background gradient on the homepage to absolute
positioning with each edge set to zero to expand to the available area.
Merge branch 'master' into bg-height
Merge pull request #64 from codeforamerica/bg-height

Fixes #47 - Make content 100% height of available area