Fixes #7. Add pan interaction

Also added a utility for detecting key modifier state on browser level events.
Suppress warning from use of bitwise operators

I considered disabling this globally, but I left it enabled for now so
it will warn us if we use one when we mean to use a logical operator.
Don't limit panning to viewport bounds
Remove unnecessary calculations
Fix drag rotation interaction on firefox

Was using a nonstandard property of mouse events.
Support mouse wheel events on Firefox (for zoom)
Fix pan interaction in firefox
Remove debugging statement
Merge pull request #10 from Kitware/pan-interaction

Fixes #7. Add pan interaction
Change mouse cursor over rendered image
attach to document's mousemove event for dragging