Merge pull request #433 from bvssvni/master

Make `AssetStore` return Err when empty
Merge pull request #434 from bvssvni/master

Upgraded to latest Rust-Empty
Removed hgl-rs triangle example

Moved to https://github.com/PistonDevelopers/piston-example-hgl_triangle

Closes https://github.com/PistonDevelopers/piston/issues/436
Merge branch 'master' of https://github.com/bvssvni/piston