Add setStubs/unsetStubs to domstubs to support testing

Do not directly export to global. Instead, export all stubs in domstubs.js and
add a method setStubs to assign all exported stubs to a namespace. Then replace
the import domstubs with an explicit call to this setStubs method.  Also added
unsetStubs for undoing the changes. This is done to allow unit testing of the
SVG backend without namespace pollution.

examples/node/pdf2svg.js

@@ -8,7 +8,7 @@
 var fs = require('fs');
 
 // HACK few hacks to let PDF.js be loaded not as a module in global space.
-require('./domstubs.js');
+require('./domstubs.js').setStubs(global);
 
 // Run `gulp dist-install` to generate 'pdfjs-dist' npm package files.
 var pdfjsLib = require('pdfjs-dist');