Add a missing parameter to commerce_round() in the line item action 'Divide the unit price by some amount.'

Issue #1288942 by rszrama: change the default price rounding mode to round the half up and add an update function that iterates over all Rule configurations on a site looking for actions that need to have this default round_mode setting set.

Issue #1288942 by rszrama: abstract our custom rounding support out of the Tax module and allow rounding to be applied on a per-unit price alteration basis; the 5 actions pertaining to manipulating the amount of a unit price now have the option to round the resultant amount using the same modes we support for tax rounding.

Add proper handling for products that don't have a price (i.e. their price amount value is NULL). These cannot be added to the cart, will not be rendered as 0.00, and will be removed from the cart if the price is no longer available for a product that was previously added to the cart.

Allow line item unit price manipulation actions to specify the price component that should be used when the difference is saved.

Include unit price changes through Rules in its components array as alterations to the base price value.

#998080 by rszrama: convert the old price altering actions to be line item unit price altering actions working through an entity metadata wrapper.

#859322 by rszrama: per fago's comments, removed any custom handling in the code for unchanged entities; unchanged entity variables now use the rules_events_entity_unchanged handler.

#966624 by das-peter, rszrama: separate line item presaving into two presave hooks that are insert / update dependent.