Adding support for media range parameters

Fix for https://github.com/NancyFx/Nancy/issues/1211. This is still a work-in-progress, but I'm opening up the pull-request to get feedback on what's been done so far.
- [x] Added a new class called ``MediaParameters`` which implements ``IEnumerable<KeyValuePair<string, string>>``
- [x] Added a ``Parameters`` property to ``MediaRange``, of the above type.
- [x] Altered ``MediaRange.FromString`` to parse the parameters into the new property.
- [x] Added ``MediaRange.MatchesWithParameters`` to take parameters into account (if parameters exist, they must exist in both, but ordering doesn't matter).
- [ ] Modify ``RequestHeaders.GetWeightedValues`` so that media range parameters are not stripped.