Add tests for the XMLHttpRequest `responseType` property

Closes #260.
Added definition for Event and EventInit
Added tests for the FileList interface
Add CONTRIBUTING.md file which contains CLA.
Modifications to CLA and disclaimer text.
Further modifications to the CLA text.
Update README according to CLA flow changes.
Tests for the Web Notifications spec
Fix issues raised by Simon Pieters
Merge remote-tracking branch 'upstream/master' into submission/ibelem/fileapi-filelist
Fix issues raised by Simon Pieters