verbosely check if shipping/billing address is null, some systems does has such value;
backward compatibility for getting product object
output magento version in universal_variable
added total_discount key in line_item according to new specification.
updated readme change log
Updated for all pages and the latest specification

All 8 page types now show UV, and include all essential parameters.
Fixed conflicts, abstracted any methods referencing models
Updated readme
All Jing's taken account for and resolved.
corrected template formatting
correct formatting; only expose visible product for listing page.
add shipping method to basket
merge branch 'master' with lastest changes; resolving conflicts

Signed-off-by: Jing Dong <jing.dong@activars.com>
Merge pull request #6 from QubitProducts/feature/spec-1.1.1

align the extension with universal variable specification 1.1.1