code fencing

[evilnick]

We should support and promote this syntax:

juju bootstrap
juju deploy something
juju do-something-else

over the indentation syntax for code samples.

1. It is Github standard
2. It means not having things break because you can't tell how many spaces you added.
3. It is already supported by python-markdown - http://pythonhosted.org/Markdown/extensions/fenced_code_blocks.html

[marcoceppi]

This is already supported since we're using GFM for generation.

Both

```
code
```

and

```lang
code
```

Are supported