Added autodetect support for pstotext an alternative to pdftotext. added...

Added autodetect support for pstotext an alternative to pdftotext. added detection for the Mac OS "Darwin" which searches the "Darwin" paths /opt/local/bin:/opt/local/sbin as well as the system paths so autodection works much better on the Mac. Also made a change so that when a new helper is added all other helpers for the fail extension gets deleted from the table so we don't have duplicates