Port to new proc macro interface

[lambda-fairy]

This changes how Maud is imported.

Users will need to change their code from this:

// Maud 0.16
#![feature(plugin)]
#![plugin(maud_macros)]
extern crate maud;

to this:

// Maud 0.17
#![feature(proc_macro)]
extern crate maud;
use maud::html;

Note the lack of #![plugin] or #[macro_use] attributes -- the new proc macro system allows for importing macros in the same way as any other item.

Closes #92