Improve australia post drc call.

Add a lookup cache (since the same lookup is repeated numerous times, from add to cart through to checkout, thus used cached values rather than a lookup each time)
Make cure the default, not file - curl is faster (see http://stackoverflow.com/questions/555523/file-get-contents-vs-curl-what-has-better-performance)

Added a cache switch to enable/disable the above.

Add a debug switch, to make debug log less noisy
Some other minor code improvements around the drc lookup