Do not shadow global XMLHttpRequest

If you use the XMLHttpRequest variable name then the if
typeof XMLHttpRequest === 'undefined' check will always
return false and the XHR polling method is never used.

This is needed because JSONP polling is buggy and doesn't emit
close when server shutsdown. JSONP should be fixed / disabled.
Merge pull request #89 from Raynos/xhr

Do not shadow global XMLHttpRequest