Add the tcod-sys crate

It uses the new build.rs script and exports the headers for libtcod.
Add dependency on tcod-sys

This removes the `ffi` module in favour of the new `tcod-sys`
dependency.
Add a .cargo/config file for binary overrides

tcod-sys/build.rs will build libtcod from the bundled source.

If you want to skip the building process (because you already built
libtcod or downloaded the precompiled binaries), you can point Cargo to
their location using `.cargo/config`.
Add libtcod 1.5.2 source checkout to tcod-sys

The build script will use this to produce the libtcod binary if it's not
specified in user's config.

Here's the specific version we imported:

    $ hg summary
    parent: 214:ff47576c3bdf
     fix compilation on latest mingw
    branch: 1.5.x
    commit: (clean)
    update: (current)
Add the OS X makefile and necessary files

Original instructions and files by Zack Hovatter from:

http://zackhovatter.com/gamedev/2013/11/26/building-libtcod-on-os-x-mavericks.html

Adopted and blogified by Jared "@jaredonline" McFarland with a makefile
investigation and fix by Paul "@pmsanford" Sanford.

Thanks everyone!
Make the build script work

Seems to work on Linux and Mac OS X.
Add the missing Windows SDL libs and upc.exe

These files are shipped with libtcod source and are required to compile
libtcod. Make sure we don't .gitignore them.
Add Windows support to tcod-sys' build script
Fix Travis

Travis no longer needs to download the libtcod binary distribution, but it needs
the dependencies to build libtcod.
Update the documentation

This mostly describes how to use tcod-rs with the new build scripts but also
fixes broken libtcod links and adds the missing contributors.