initial support for multiple background images

Adding code from http://jsbin.com/ebigux/latest
satisfy lint task
style update
move parseBackgroundImage to Util; add tests
parse out prefixed methods
export html2canvas, pass value not def to backgroundImage
rewrite parseBackgroundImage to remove RegExp

RegExp couldn't handle parens inside parens:
linear-gradient(rgb(0,0,0),rgb(255,0,0))
fully remove regex; simplify whitespace check

jshint complained about case statements w/o break (boo)
don't generate gradient on invisible elements
replace definition property, w/ args[]

Also fix reference for preloader
update test names
refactor Preload
initial support for backgroundSize
Merge remote-tracking branch 'niklasvh/develop' into develop

Conflicts:
	src/Core.js
	src/Parse.js
	src/Util.js
implementing background-size; use parseBackgroundImage in render
working background-size
backgroundSize needs trimmed before second split

also move trimText to Util
trimText regexp needs /g
add tests for resizeBounds
remove todo comment