RFC: Documentation System

Ok, so this has ended up evolving into a full doc system. Functions and macros can be documented with
```julia
@doc """
Description here
""" ->
function foo() ...
```
(prettier syntax forthcoming)

Documenting multiple methods of a function will concatenate the doc strings together, in the order the methods were originally defined.

By default `@doc` treats unadorned strings as equivalent to `md""` (i.e. markdown syntax) but you can actually put anything before the arrow to associate it with the function.

Docs can be queried with e.g.
```julia
doc(Docs.doc)
@doc Docs.doc
@doc @doc # For macros
@doc text""
```
(and you can try these out on this branch)

---
*Original:* 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.
In the abovementioned issue, Michael wrote:

> Docile 0.1.0 parsed all the docstrings into Markdown.Blocks. This led to a slow down in the load time of modules I was testing on since Docile had to import Markdown.jl and then parse each string as well. This won't be a problem once pre-compilation is available, but for now it's a bit annoying.

> 0.2.0 now just captures the strings and leaves the parsing and rendering business to Lexicon. This has made loading modules that use @doc quite a bit faster (just anecdotal evidence though, I've not got any numbers regarding this).