Added word histogram GUI.
Added function to build word histogram and create its GUI.
Fixed histogram (building from text in tab not in the popup).
Failing gracefully (UI msg for user) if text in tab can't be read.
Fixed histogram initial count.
Bar chart img normalized relative to max count.
Minor CSS changes.
Implemented word swapping.
Added TODO.
Fixed word swapping.
No multiple swaps (aaa->bbb + bbb->ccc != aaa->ccc).
Respect casing.
Cleaned up content script, added traverseText method.