Added global variable for png window

Changed the new window to point to a blank page that has the png written
into it instead of pointing the new window to the data URI directly.
Chrome doesn't like the protocols (http vs data URI) not matching when
trying to read the contents of the new window.
Added export PNG test spec; renamed path.js to draw_path.js
Spec names update
Merge pull request #100 from tranek/develop

Added Export PNG test spec