fix issue #13
admin check for readable piwiktracker.php file
more checks for piwik_enable setting in main piwik.php file
minor other changes
Merge branch 'develop' of https://github.com/InaneCoding/Piwik-OpenCart-Ecommerce-VQMod into develop

resolving conflict arising from commiting with "amend last commit"!
Conflicts:
upload/admin/language/english/module/piwik.php