Add Short GUID support (#3)

[danieljsummers]

There are three different commits here (initial PR). I did them this way because I did 3 different things:

• The first updated the dependencies against the latest Giraffe (it was still targeting 2.0.x) and fixed the unit tests that change broke (Giraffe now put the character set as part of the content type).
• The second adds in the short GUID support, by simply calling out to Giraffe's implementation. It's one if statement on the GUID parsing logic, so hopefully that won't slow things down. I added a test for this third GUID format.
• The third adds the HEAD and OPTIONS route groupings; this will provide a workaround for TokenRouter needs explicit GET when POST is used #1 (oops - think I said number 2 on the commit) in cases where the only reason they aren't under a verb is that one of those needs to be mapped. (This is an actual use case I have.) I also added a test for each verb to make sure that they worked as advertised.

At any rate, feel free to squash and/or cherry-pick as you see fit. Also, if something needs to be done differently, just let me know.

[danieljsummers]

I'm out of time for this evening. The dotnet version is what's throwing these builds off. Giraffe 3.4 is the last version to target 2.1; let me know if you want me to rework the deps and target the latest available on the dotnet SDK 2.1 series.