Refresh Bids API call

[Heray]

As a site that caches pre-bid calls, I'd like a new API function to request new rounds of bids for all my bidders, so that my site won't have latency for pre-bid calls.

This is in progress. Our current design:

requestBidsForAdUnit(adUnitCode)
A non blocking function that will send out bid requests for the given ad unit's bidders. If adUnitCode is not given, the function will request bids for ALL ad units.

More details:

• Re-parse all configuration settings on calling requestBidsForAdUnit() function
• If they call for a particular placement, only parse settings for that placement
• Remove all existing bids
• Time out existing bids that are pending
• Call for new bids for all or given placement

[mkendall07]

Created requestBidsForAdUnit(adUnitCode) function on master