API Fixed CartCleanupTask, define time in mins rather than relative

Relative time is prone to errors. Case in point:

With a current date of 2014-05-27 13:05:21

php -r 'var_dump(date("Y-m-d H:m:i",strtotime("-2 hours")));'
string(19) "2014-05-27 11:05:21"

php -r 'var_dump(date("Y-m-d H:m:i",strtotime("-2 hrs")));'
string(19) "2014-05-28 03:05:21"

This would delete ALL of your pending orders because of a typo.

Also fixed the task itself, it plainly wasn't working (undefined $cart etc).
Didn't want to introduce a mocking library into the project, so written
a small fake as a simple subclass in unit tests.
Merge pull request #252 from silverstripe-iterators/pulls/cart-task

API Fixed CartCleanupTask, define time in mins rather than relative
Select Order.Paid field when using having in ShopPeriodReport.
Fixes #253
Add a little flexibility to PopulateCartTask
Added 'payablecart' fixture to yaml Orders
Don't recalculate order when placing order, as this is a post-payment state where the order total should not change.
Allow products to be 0 price.
+ Tidy and testing.
Allow orders to be placed with $0 value
Fixed SQL case sensitivity in CartCleanupTaskTest
Merge pull request #254 from silverstripe-iterators/pulls/fix-cartcleanuptest

Fixed SQL case sensitivity in CartCleanupTaskTest
Restore postgresql compatibility
FIX: Removed unnecessary 'setWhere' that was breaking ShopSalesReport
Removed some old, unused code from ShopPeriodReport and ShopSalesReport. Still need to get pagination working.
Prevent negative order calculations, and log cases where this happens.
ENHANCEMENT: Allow order to hook into validate data on checkout
ENHANCEMENT: Creating an order instance should use the Injector
Added 1.0.x-dev branch alias to composer.json
Merge pull request #256 from milkyway-multimedia/patch-1

Be able to replace the order class
Merge pull request #257 from markguinn/patch-1

Added 1.0.x-dev branch alias to composer.json
API:  Added category functions to Product for fetching categories, or category ids associated with a  product.