Add MultiValueDictionary constructor with key comparer parameter

[majerm]

MultiValueDictionary implementation has ability to define value comparer, but it is not posible to define both - key and value - comparers. This pull request chanes it.

[FransBouma]

I said earlier I'd push a new version to nuget but that will be a bit problematic as nuget of course enforces a new version number. So we'll postpone pushing this change to nuget to the near future if we have further changes to add. Thanks for the contribution :)

[majerm]

I end up with more modifications and now I use more modified class inherited from SortedDictionary and inner SortedSet. So release is not priority for me :)