Push checks for jQuery at the end of the stack

In most of my projects I'm including JavaScript at the bottom of the page.

Since DebugKit injects its content into the <head> tag, I noticed that it also added jQuery even though the version used is compatible.

This commit pushes the checks for jQuery at the end of the stack allowing page content to be loaded, including script tags at its bottom. It also changes the XHR call to be asynchronous (in Firefox you get a warning using synchronous requests).