Removed re-usability from jXHR object (no more open, send & onreadystatechange suppor...

Removed re-usability from jXHR object (no more open, send & onreadystatechange support). Streamlined the implementation and put it back into ajax.js (removed xhr.js in the process). Went back to a more simple & direct approach to options handling (keeping much room to further minification-related optimizations). Code is cleaner, smaller & faster. Removed & edited unit tests accordingly. All build files have had xhr.js removed.