Add support for non-Github repository services.

[jgrosso]

See elm-lang/elm-package#188.

[jgrosso]

Do note that I can't get the build to run (Cabal dependency problems), so I don't know if it compiles (and, obviously, Travis CI is having rake errors). I'm working on that now and will update accordingly if no one else can test it.

[JoeyEremondi]

It's a lot more complicated than this.

The following modules will break because of the change:

• https://github.com/elm-lang/elm-package/blob/master/src/GitHub.hs (assumes repos are GH)
• https://github.com/elm-lang/elm-package/blob/master/src/Install/Fetch.hs (assumes repos are GH)
• https://github.com/elm-lang/elm-package/blob/master/src/Publish.hs#L84 (will publish based on user and name, even if repo isn't GH)

The restriction to GitHub isn't arbitrary right now, it's because elm-package depends on the GitHub API to work.

Any accepted PR should at least ensure that it is impossible to publish non-GitHub projects.

[jgrosso]

@JoeyEremondi Duly noted. Thanks for the clarification.