add bidPriceExceeded function for multiple currencies support

`USD_CPM` currency is hardcoded in `RTBKIT::Router class`. As discussed there: https://groups.google.com/a/rtbkit.org/forum/#!topic/discuss/bVFv0Dh6kx4:  it can be a problem when we want to add new currency. I create `bidPriceExceeded` function and move bid price comparition there. It's virtual function and we can override it and set new bid price range.