Fixes command-line issues e.g. running unit tests

Fixes #443.
Extract output buffering to function.
Use regular buffering if gzip option not set.
Use buffering in AJAX requests, to avoid “headers already sent” warning
in some situations.

No longer supporting PHP 5.1.
Fixes #394

And now we have local Joomla integration, we don't have to fail if the Joomla plugin isn't installed.