Add bid optimizers

This patch adds the ability to use existing bid optimizers in an ad
group or create a new one from a bid optimizer engine.
Add bid optimizers in library

This patch adds the ability to list / edit one bid optimizer.
It lacks some features, like editing properties, but the demo is this
afternoon and I need to push that now !
Merge pull request #9 from dduponchel/bid_optimizer

Bid optimizer