refactor(headers): Use u16 based newtype for quality value

Using floating point numbers is problematic because comparison is inexact.
They also take more space than integral numbers in this case.

Add `FromPrimitve`, `ToPrimitive` and `Default` traits to quality newtype.

Closes: #330

BREAKING_CHANGE: Replace f32 quality values in quality items with a
Quality(u16) newtype. Valid values are from 0 to 1000.

src/header/shared/mod.rs

@@ -1,6 +1,6 @@
 pub use self::encoding::Encoding;
 pub use self::entity::EntityTag;
-pub use self::quality_item::{QualityItem, qitem};
+pub use self::quality_item::{Quality, QualityItem, qitem};
 
 mod encoding;
 mod entity;