Format version only if it exists

An App has no version when it is first created. Format the version only
if it exists (has been returned from the server).
ThreadPoolContext: global execution context, which is backed by a cached thread pool.
Replaced all occurrences  of global execution context to the thread pool context.
Merge pull request #328 from mesosphere/wip-separate-threadpool

Replace global ExecutionContext by a cached thread pool context
use sbt instead of maven
update versions
added scalariform and reformatted source
fix bin/start and README
Merge pull request #269 from drexin/wip-use-sbt-drexin

use sbt instead of maven