Provide a way to separate apart a project into multiple build stages

[bguthrie]

Right now, CC.rb has no way, either visually or in the config, to break a project's build into multiple stages. Multiple builds that share the same SCM are separated using an "if"-switch in the project config, and although one "project" can depend on another, there's no visual indication of those relationships.

In order to build anything even remotely resembling a proper build pipeline, it would help if there was a nice way to both show and define this information. CC.rb becomes difficult to manage once you've defined a lot of separate projects.