4.0.0: Application initialization & tracing

• Heads up: Removed deprecated transfer() methods from Stream,
  Upload and Response classes, they have been superseded by transmit().
  (@thekid)
• Merged PR #91: Application initialization. Implementations can chose
  to implement the initialize() method (which is empty by default)
  to perform database migration, wait for dependant services, etcetera,
  when the server starts up.
  (@thekid)
• Merged PR #103: Make it possible to append trace data to the response
  which will appear in the log file
  (@thekid)
• Merged PR #102: Extract static content handling into reusable class.
  See also xp-forge/frontend#39
  (@thekid)
• Merged PR #89: Add optional parameter $append to cookie() - @thekid
• Merged PR #107: Fix SAPI uploads with array parameters - @thekid