add an `include_str!` example

[mutantbob]

I added the include_str! example we were talking about. I also removed some unsafe keywords that the compiler said were unnecessary. I know it has demanded the unsafe keyword in other instances of an unsafe function body, but I don't know all the subtleties.

[Cryptjar]

The doc comments look good, but I would appreciate keeping the unsafe keywords, in fact, I was planing to enable:

#![feature(unsafe_block_in_unsafe_fn)]
#![deny(unsafe_op_in_unsafe_fn)]

However, there are a few unsafe fns that need even more unsafes for that, that's why I haven't enabled it yet. It's on my to-do list.