Add default price component handling for price fields.

This issue is related to #1124144: how to setup drupal for a school scenario？.

In the issue that I linked to above, Ryan has made a very good fix to the whole problem, that if you change a price field in the UI the price component won't get update. Since a lot of stuff is going on with the price you usually want to change: the line item unit price, a lot of special care is needed for price field, since we might have taxes, discounts etc, that could get effected by the price change.

The problem is that every price field that is added by the users won't have these price components. This is especially needed when people start making custom line items, that should be added to orders. Right now we require that they know a lot about how the internal price handling is done for commerce. I have done a lot experience in this area since the early alpha of commerce, and I think we could help developers a lot by adding some default price handling.

Another benefit of this, is less code to maintain. To set a value on an empty price field, you would simply set the amount and currency code. To change a price, you would change the amount and let the price field handle the rest. This simple approach can't handle all cases, as we've seen in the above issue, but providing a default handling that just works for most cases, seems like a good addition to me.