Add missing constants

Constants should be available on the instance as well (http://www.w3.org/TR/XMLHttpRequest/#interface-xmlhttprequest)
Throw on trying to open blacklisted methods

http://www.w3.org/TR/XMLHttpRequest/#the-open-method
Merge pull request #8 from webitup/master

Thanks, Mangled!
Fix typo and remove trailing space.
Merge pull request #9 from HelderMagalhaes/master

Fix typo and remove trailing space.
Issue 10 - window.document.all is not good enough to detect IE
Merge branch 'master' of github.com:ilinsky/xmlhttprequest
Added type indicator to variable