Allow negative amounts for price set fields / options

As discussed in this forum post, there are cases where it's useful to create price set fields and / or field options with negative amounts:
http://forum.civicrm.org/index.php/topic,9594.0.html

1. Modify form rules for adding / editing price set fields and options to allow input of negative numbers.
1a. Add explanatory text to price set field help: "You may enter a negative amount value if the field or option is being used to discount the total event price."

2. Add form rule to online event registration form (Event/Form/Registration/Register) to prevent user from submitting if TOTAL of price set amounts is < 0.

3. Add form rule for backoffice event registration to prevent user from submitting if TOTAL of price set amounts is < 0 AND the we are in "credit card transaction" mode. (i.e. allow negative total amount if we are NOT submitting the transaction to a payment processor).