Refactoring of tests and renaming of classes

Split tests into multiple test files.
HeaderAcceptValue is now MediaRange.
HeaderAcceptList is now HeaderAccept.

Attributes quality and options are now read-only property, and underlying information are still available into _quality, _options and _raw_options.

When serialize into a string value suitable for an HTTP Accept header media-range, the parameters are now ordered, and q is always first.