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.
I agree, but absolutely nothing about that comment makes it clearer. In an implicit, the in and out tell you what the conversion does. The comment here is literally a repetition of the function signature: name (`Lift`), input type (`Java Func0`), output type (`Scala Function0`). The class documentation mentions that the implicits within are meant for bridging between Java and Scala functions… Maybe I could expand that class-level doc a little instead?

I'm also not sure the terms `lift` or `drop` really offer any clarity either (the domain of the functions in question isn't really changing, and while strictly speaking it can still be considered a lift, using that cryptic term doesn't give convey anything useful IMO).