Fixes #193 - masks popup arrow

The arrow on the popup is made by rotating a box 90 degrees. The
consequence of this is that the lower corner of the box overlapped on
the text when the popup was scrolled. This crops the arrow so that it
forms an actual triangle and doesn't overlay on top of the text when
scrolling.
Moves popup up slightly

Moves the header bar popups up one pixel to eliminate blue line that
was spilling over from the background above the popup box.
Adds code documentation to the popup handler
Merge pull request #207 from codeforamerica/popup-arrow

Popup arrow