Merge pull request #12 from Coeuvre/master

Added `remove_observer` to back-end
Updated links to docs
Merge pull request #13 from Coeuvre/master

Rename event context to follow the discussion of #10
Merge pull request #14 from Coeuvre/master

Added `AddRelease` and `ReleaseKeyboardEvent`
Merge pull request #19 from Coeuvre/master

Renamed 'Map' to 'Call'
Merge branch 'master' of https://github.com/PistonDevelopers/rust-event