Implement some convience traits for primative types.

[a2aaron]

This PR implements Add, Sub, AddAssign, and SubAssign for the basic integer primatives (u4, u7, u14, u24, and u28). I find this to be useful when working with things such as ticks, which are usually encoded as u28s.

This PR also implements From<Fps> for u32 and f32. This is simply for convenience, since there are already as_f32 and as_int methods on Fps.

[kovaxis]

Great! Thanks!