change the initial background from white
allow chains and residues to be passed to viewer.fitTo

Per suggestion by @Traksewt.
Merge pull request #42 from Traksewt/fix-original-background

change the initial background from white
add section of color notations to docs
use forceRGB for background color option
Merge branch 'master' of https://github.com/biasmv/pv into create-mouse-down-events
adding doc and refactoring event framework