FIX PHPUnit latest not working with composer installed builds

When using composer, we must rely on the composer autoloader to
load in PHPUnit and not try do to so ourselves, as the old
PHPUnit\Autoload.php file doesnt understand how to find things
in vendor
Merge pull request #1118 from silverstripe-rebelalliance/fix/phpunit/3.1

FIX PHPUnit latest not working with composer installed builds
BUG Exception handling and email notification mechanism now correctly considers the stacktrace as provided by the exceptionHandler function, instead of attempting to perform a debug_backtrace further down the reporting chain (which ends up generating an unnecessarily nested stacktrace). Debug was cleaned up so that errorHandler and exceptionHandler both act consistently. As a result, the LogErrorEmailFormatter class could be simplified.

This was required to fix a bug in which exceptions would not have a visible stacktrace when handled by the email logger.