Merge pull request #27 from bvssvni/master

Added `Usage` enum
Merge pull request #29 from bvssvni/master

Added `Get` and `Modifier` impls on `Usage`
Merge pull request #31 from bvssvni/master

Added `Get` and `Modifier` for `RefCell`
Merge pull request #33 from bvssvni/master

Changed to `Modifier<&'a RefCell<F>>`
Merge pull request #35 from bvssvni/master

Changed impl `Get` to `&RefCell`
Got rid of Modifier constraint on Set

Closes https://github.com/PistonDevelopers/current/issues/39