Add support for Gigahorse http library.

[alexdupre]

I've kept dispatch as default. I'm not sure if we prefer that scalaxbGenerateGigahorseClient automatically disable the dispatch client; currently it doesn't, so you have to set scalaxbGenerateDispatchClient := false and scalaxbGenerateGigahorseClient := true to generate the Gigahorse clients.

[eed3si9n]

wow. Thank you very much for this! Instead of Booleans what do you think about defining sealed trait HttpClientType or something like that?

[gabro]

Cool! Does this enable supporting scala 2.12 as per #408?

[gabro]

@alexdupre 🎉 wonderful, thanks!

[alexdupre]

@eed3si9n: would you prefer to replace scalaxbGenerateDispatchClient, scalaxbGenerateDispatchAs, scalaxbGenerateGigahorseClient (or just dispatch/gigaorse client) with a single key of HttpClientType? Or such additional key, if set, would set the value of other three?

[eed3si9n]

I think for backward compatibility we should keep any existing keys, so the second option I think would be better.

[alexdupre]

I'm about to push a commit for it that simplifies the sbt setting for a single client, but I'm sure you have much more knowledge on sbt, so feel free to replace it completely as you prefer.

[eed3si9n]

Sounds good. I'll merge this as is, and change it around later if I have to.