improved handling of stock options to save changes to a variable product
stopped endless loop when saving variable product that manages stock

@mikejolley
fixed variation tax_class and backorders

@mikejolley
fixed undefined offset when added products in cart
adding helper tooltips to system status - fix #5786
Fixed issue #5823
Fixed issue #5841
following wc php coding standards
Moving system status helper tips on the left of the value
Merge branch 'master' of https://github.com/woothemes/woocommerce
added default behaviour for currency position in reports