MathJax widget - same origin policy issue

[Piotr Jasiun]

Make MathJax plugin works when document.domain is set.

[Piotr Jasiun]

Problem solved. This ticket branch is part of ​t/10937.

There are no automatic tests. To test this ticket:

• define sub.ckeditor.dev host to point to the same folder as ckeditor.dev,
• open ​http://sub.ckeditor.dev/plugins/mathjax/dev/mathjax.html
  • check if there is mathematical equation between "Foo foo" and "bar bar",
• open "Math" dialog,
  • check if preview appears,
• create new widget using dialog,
  • check if it appears.

This ticket should be tested especially in IE and FF.

[Piotr Jasiun]

• ​git:95bd2fe
• tests:b802386