Support for XMLHttpRequest mocking libraries

Rather than capturing and reusing the window.XMLHttpRequest at
the time of load, XMLHttpRequest will instantiate a new window.XMLHttpRequest object at run time.  This allows
mocking frameworks (e.g. sinon.js) to temporarily mock the object
in unit tests.
Fix incorrect timestamp format in the fake readystatechange event
Merge pull request #15 from elerch/master

Support for unit test mocking of XMLHttpRequest
Merge pull request #17 from sammyhk/master

 Issue #16 - Fix incorrect timestamp format in the fake readystatechange event
Issue 6 - fGetDocument error for IE
Updated text with a note on obsoletion and recommendations
Minor text edits