Updated README, fixes #154
Added Retina support

Closes https://github.com/AngryLawyer/rust-sdl2/issues/164
Updated to latest Rust
New syntax for Cargo.toml lib
Update Travis CI
Annotate all SDL structs
Run codegen script outside of make
Build demo via cargo
Changed target_os from "win32" to "windows"

As of rust pull request 16435, cfg attributes now use "windows" instead
of "win32" for the target_os parameter.
Update Travis CI
remove glob using in response to compiler change
remove commented out code
allow non_snake_case for new rust lint behavior
replace deprecated get_ref call
Add simple message box
Added missing library path
Remove unnecessary transmute() calls, prefer "as" instead
Match bitflags! macro style in latest rust
Update readme for cargo build method
Add link to example repo