Move to rustdoc instead of ruby's middleman

* All markdown files are now rendered with `rustdoc` into HTML
* A JS syntax highlighter, prism, is included for TOML syntax highlighting.
* A new makefile target, `make doc`, will build docs into `target/doc`
auto merge of #550 : alexcrichton/cargo/rustdoc, r=brson

* All markdown files are now rendered with `rustdoc` into HTML
* A JS syntax highlighter, prism, is included for TOML syntax highlighting.
* A new makefile target, `make doc`, will build docs into `target/doc`

preview: http://people.mozilla.org/~acrichton/crates.io/doc/

Closes #524