friendly DSL

[jastice]

Right now adding a source dependency is a bit unwieldy:

sourceDependencies += Dependency(
        HashVersion("<git commit long hash>"),
        Repositories(uri("<https git repository uri>")))

We'd like something more like:

sourceDependencies += hash("<git commit hash>") from uri("<repo uri>")
sourceDependencies += tag("<git tag name>") from uri("<repo uri>")
sourceDependencies += branch("<git branch name>") from uri("<repo uri>")
sourceDependencies += hash("<git commit hash>") from Seq(uri("<repo uri1>"), uri("<repo uri1>"))
[etc...]