Add Markdown.jl

Ok, here's a first pass at getting Markdown.jl into Base. This will us allow us to display Julia's doc strings in the terminal and rich environments; see the [readme](http://github.com/one-more-minute/Markdown.jl) for some examples.

It could probably do with some exports, e.g. the Markdown module and the `readme()` function would probably be useful.

Doc objects can be created by the `doc""` string macro, which is just like `md""` with some subtle differences. This isn't 100% complete yet, but in future `doc` will deal with things like handling image URLs and lazily evaluating interpolated code. All that's then needed is to associated the returned `MD` object with some function, which is easily done.