Fix PassengerRestartDir functionality

Code in HEAD requires PassengerRestartDir to be an absolute path. This patch restores old functionality allowing both relative and absolute paths.
Disallow USE_CCACHE when running the debian:production task.
Move all build output into a single directory: buildout
Allow Passenger Standalone commands to require libraries before they are run.
Fix 'passenger package-runtime' command
Correctly mark a RequestHandler client as disconnected. Fixes issue #889.
Document USE_CCACHE in CONTRIBUTING.md.
[ci:skip]
No longer crash when the Phusion Passenger native_support Ruby extension cannot be compiled.
E.g. because the Ruby development headers are not installed or because the current user has no
permission to save the native extension file. Fixes issue #890.
Replace references to lock_guard to boost::lock_guard to fix compilation problems on OS X 10.9. Fixes issue #906.
Do not crash when the native_support build system fails.
Remove dependency on bash.
This way, Phusion Passenger works out of the box on BSD platforms without installing/configuring bash. Fixes issue #911.
Fix 'PassengerPoolIdleTime 0' not being respected correctly. Issue #904.
If the server instance directory already exists, it is now removed first in order get correct directory permissions.
If the directory still exists after removal, Phusion Passenger aborts to avoid writing to a directory with unexpected permissions.
Fixes issue #910.
When determining the C++ binary compatibility ID for non-OS X systems, do not query cpp anymore because we don't want the ID to depend on installed software.
Add documentation describing how to compile a custom Apache.
[ci:skip]
Change the Passenger Standalone support directory layout to match the new buildout layout.
Fix Passenger Standalone inability to start up because of the previous commit
Fix a Passenger Standalone regression introduced by the server instance dir security change.
Locations.ini was being deleted. The Passenger Standalone temp dir is now a separate directory
again, but touched every 30 minutes to prevent /tmp cleaners from deleting them.
Fix bugs in the build system
Allow customizing Union Station send intervals.