Merge pull request #135 from nwin/master

Fix for #133
Merge pull request #136 from nwin/master

Fix deprecation warnings.
Upgraded to latest Rust

Closes https://github.com/PistonDevelopers/image/issues/137

* Added Primitive placeholder trait
* Added `max_value` to replace deprecated Bounded
* Added imports to Float, SignedInt and FloatMath
* Removed Num constraint on clamp functions
Merge pull request #141 from bvssvni/master

Upgraded to latest Rust
Always read the background field.