fuse is not mentioned in the book as opposed to the README in redox repository

The `bootstrap` script works as well on MacOS X

Add `override` statement

Add an additional description of how to install the nightly version of Rust, using `rustup`.

`multirust` is now deprecated - now use `rustup`

The changes include:
- Editing for flow and understandability.
- Unifying how links are included in the source files.
- Adding an `About this Book` page to the introduction.
- Adding a few examples or new pieces of information I felt were
  lacking when I read the book.
- Unifying 'user space' and 'kernel space' spelling.
- Added myself to the authors list in `overview/welcome.md`.

See http://azerupi.github.io/mdBook/format/theme/syntax-highlighting.html