fix bug in Crafty.DOM.translate

Arithmatic error: The translation was incorrect when the viewport scale
is not 1 AND the viewport x or y is not 0. This fixes it. Also improved
the documentation.
add test for Crafty.DOM.translate()
fix test of Crafty.DOM.translate()
Merge pull request #701 from sbyrnes321/zoom_error

fix bug in Crafty.DOM.translate when viewport scaled+zoomed