Add HHVM module

* Provision separate set of config files for CLI and FastCGI modes.
* CLI configs in the default locations (so that running 'php' with no arguments
  "just works"); FastCGI configs in /etc/hhvm/fastcgi.
* Separate bytecode caches for the two modes.
* Make start-stop-daemon manage the pidfile so Upstart can inspect it in a
  post-stop stanza.
* Make Upstart check for a stacktrace in post-stop and invoke
  $HHVM_TRACE_HANDLER (if defined) to process it.
* Add a prototype Python script for parsing stack traces and reporting them to
  Sentry.
* Manage /usr/lib/hphp/extensions/current symlink pointing to the extension
  directory for the most recent ABI version.

Change-Id: Ieadc53775978a43f45008522fbc574f80c66e306