v0.7.0-beta4: Almost done(), asynchronous postBuildTasks & bundleBuilder instance resolved

BundleBuilder.buildBundle :

• promise always returns bb instance
• runs done() async tasks serially, passing (err, bb) or (err, bb, callback) or compatibility (true/false) with older grunt-urequire / urequire-cli
• specs for done() tasks, then (bb)->, failing catch(err)