Remove enableGc from LiftJavaScript config.

We now decide whether or not GC is enabled by the presence
of a data-lift-gc attribute.
Redo body attribute calculation.
Move page-specific JS into a RequestVar.

The RequestVar contains a JavaScriptResponse, though it
isn’t yet hooked up. Looking up /lift/<page id>.js will return the
file in that RequestVar, if present.
Replace ajaxPath/cometPath with liftPath.

liftPath is now the root of Lift requests, and everything under
it is under our control. AJAX lives at $liftPath/ajax and comet
at $liftPath/comet. Additionally, we drop ajax and comet from
the JSArtifacts. These functions are now provided directly by
lift.js.
Drop ScriptRenderer, which is no longer used.
Move LiftMerge#scriptUrl to liftPath.
Namespace page script under $liftPath/page/*
Fix compile error with page inclusion check.
Fix construction of body attributes.

We had switched to half-String half-Option[String], breaking
compilation; it’s all String now.
Added Promise code to lift.js