Update README.md
Replaced all fail! macros with panic! macros for compatibility with https://github.com/rust-lang/rust/pull/17894
Merge pull request #125 from burtonageo/master

Replaced all fail! macros with panic! macros
Updated example to use panic! macro (omission from https://github.com/PistonDevelopers/image/pull/125)
Merge pull request #126 from burtonageo/master

Updated example to use panic! macro (omission from PistonDevelopers#125)
Merge commit '6dc22cb810c83f16c7fbc8e098275baabe216be1'