Singularity 0.4.0

Very incomplete list of changes:

• include rack information when communicating with Baragon
• add grep functionality to log tailer
• use Horizon http client
• tweaks to how SingularityExecutor stages the filesystem of a task
• email tweaks
• reconcile tasks using mesos framework API
• many tweaks to Guice usage
• SingularityClient cleanup
• vagrant work/fixes
• UI fixes
• doc updates