v0.4.0

• BC break: Separate launch() and launchZen().
  BaseZen now exposes a Promise instead of being a Promise.
  (#34)
• BC break: Improve close() handling, now resolves and cleanly terminates process
  (#33)
• Now uses react/child-process and no longer depends on any extensions
  (#32)
• Support React v0.4 (while keeping BC) and first class support for HHVM
  (#35 and #36)