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.
Is Markdown.jl performing poorly enough to impact load times for you? I'd love to see the benchmarks that led you to that conclusion, since I'm not able to reproduce.

Base currently contains 1296 doc strings with an average of 4 non-blank lines. Let's be optimistic and imagine that Markdown.jl's elegant output inspires a cascade of flowing prose from Julians everywhere, such that Base's docstring count quadruples overnight *and* each one is ~10 lines long (including code samples, headers etc. of course).

On my machine, parsing 100,000 such docstrings takes all of 7.43 seconds – in other words, I can parse 13,460 doc strings per second. You'll have to forgive me for not optimising at all yet, but it's not too shabby either. By our estimation above it would add about 0.37 seconds to Base's load time.

Last time I checked (just now, that is) loading Base's 70,912 lines of code takes two minutes, which means the overhead worst (best?) case is 0.31%. To an extent these things are subjective, of course, but that strikes me as a premature optimisation. I'd much rather spend my time making Markdown.jl itself faster.

Again, though, I'd love to see your benchmarks, just to make sure I haven't missed anything.