Permission checking hook

When I opened this issue, I only had a vague idea of how Commerce Auction works and how I wanted to extend it's functionality. It does not implement a hook.

For situations where the bidder has an account balance either in credits or currency, this patch allows you the option of verifying that there is enough credit in the account to cover the cost of the bid before it is accepted by supplying the name of a function that gets the user's current balance using the current user ID as an argument. I use this with the 'Balance Tracker' module but any function in any custom module will work.