Changed context menu text to 'Translate with BabelFrog'
Added TODO: consider opening up settings page on install
Refactored BabelFrog.activate and BabelFrog.eventListener
Fixed errors in BabelFrog.expandToWordBoundary

It had an off-by-one error with endOffset calculation.
Added comments and simplified code.
Fix bug in BabelFrog.expandToWordBoundary;

Happens when selecting a whole <li> via triple click. Then
endContainer.nodeValue is null.
Clicking expands tooltip to presents linguee links.
Pressing escape dismisses tooltip.