Clarify fakeXHR documentation

Regarding @spectras comment on #22, we can improve our fakeXHR documentation.

Here are some gotchas that might find their way into the documentation by the way:

• I added fakeXHR.requests = [] to my QUnit module's setup to ensure my tests are independent. Otherwise, I might end up responding to requests from another testcase. If fakeXHR.requests is considered as an internal api, maybe it would be useful to add an accessor method?
• I use requireJS:
  • I looked for fakeXHR for a while before I realized I had to add it as a requirement (define(['fakeXHR', 'dexter', ...] function (fakeXHR, dexter, ...) {).
  • to have fakeXHR work fine it needs a fakeXHR: "dexter" entry in the path configuration so it knows it is loaded from the same file as dexter.
  • apart from that, dexterjs plays surprinsingly well with requireJS.

Start by locating the fakeXHR documentation and the RequireJS usage examples referenced in the issue. Document request-state isolation, the fakeXHR dependency and path configuration, and what is already known about the internal requests API; done means these gotchas are clearly covered.