Adjust header to work on mobile. Closes #69

Added a breakpoint, using mobile first and adding a breakpoint for greater
than 600px. This adds a colored background for region so it overlaps the
other text gracefully. Makes title text smaller and removes top margin.
Moves search box down and slides other content down accordingly.
Add breakpoint for tablet sizes
Merge pull request #71 from squareleaf/header_mobile_styling

Adjust header and search box to work on mobile. Closes #69