Load a specific version of jQuery

Sometimes you need a specific version of `jQuery`. Here you go! This snippet adds
- a global variable (`JQUERY_VERSIONS`) that contains all versions of `jQuery` (excluding `beta`s, `a`s, and `b`s)
- a global function `loadjQuery(version)` that loads the specified version or latest if version is not in `JQUERY_VERSIONS`.
- a global function `resetjQuery()` that restores the state of the scope just before `loadjQuery` kicked in.

It appends only one script tag to the body no matter how many times it is called. Most of the time it doesn't manipulate the DOM if the version you want to load is the version already available.