Merge pull request #22 from Coeuvre/master

Removed `BackEnd`, Added `EventCenter`
Merge pull request #33 from Coeuvre/master

Separated rust-event and piston. Used generic key type. Added `CallOnce`, Added `AddAny`
Merge pull request #34 from Coeuvre/master

Added `EventGame` to integrate with Piston, Added a parameter '&mut App' to command closure
Merge pull request #35 from Coeuvre/master

Added `AddAll` and `AllEvent`, Updated EventGame loop logic
Merge pull request #36 from Coeuvre/master

Added `AddAfter` and `AfterEvent`, any event context that is `Triggered` can use `after`
Merge pull request #41 from Coeuvre/master

Changed lifetime of closure to `'a`
Added expression example
Merge pull request #47 from bvssvni/master

Added expression example
Fixed 'README.md'