Doc, Stock and Two Smoking HTML Tags: Cleanup pass in lift-common scaladocs

Hopefully I'll get to do this with the other modules in short order as well. This pass
removes HTML from scaladocs, adds wiki markup in its stead, adds wiki markup
in places where we were using just text, adds examples here and there, and generally
tidies up the documentation.

It also adds the ability to link to Scala's APIs and `scala-xml` APIs from our scaladocs,
which required some tweaks to the build.

In the process, also did some very slight code cleanup. Each of these is split out into
a commit in case we want to selectively remove it. Of particular interest is that I
removed documentation that seemed to add nothing on top of the existing function
name or signature in https://github.com/lift/framework/commit/55fa831d9b848c5d37d376f56fa2a4ec82f2d8e7. Let me know if you think that's a problem.
Need to link in `EmptyBox`.