Documentation for Meddle

I haven't been paying  much attention to the details of your webstack construction, and I realized last night (when I wanted to make it serve a specific file) that I didn't know what was going on.

I'm also not aware of any real documentation beyond uncommented examples.
I've added some comments to Meddle.jl that I *think* explain what these "middleware" things are/do. It is probable that I've misunderstood some of it, so please correct me.

I'm not sure this is the best place for this documentation, since annotated examples of using it is better. With sufficient annotation of our existing examples, a note in the README about how to actually run the examples might suffice as a tutorial.

This kind of documentation should really show up in the REPL in `help`, but I'm not sure what we'd need to do to make that happen. (Can packages add to the help/documentation database that the local REPL uses? @StefanKarpinski)