Move pagevisibility/ to page-visibility/ to match /tr shortnames.
Add tests for the XMLHttpRequest `responseType` property

Closes #260.
Merge pull request #231 from tobie/fix-page-visibility

Move pagevisibility/ to page-visibility/ to match /tr shortnames.
Merge pull request #260 from mathiasbynens/xhr-responsetype

Add tests for the XMLHttpRequest `responseType` property
Added definition for Event and EventInit