Add support for Google Adwords Remarketing features in Google Analytics

Google Adwords Remarketing campaigns require the use of different Google Analytics code which enables a third-party cookie from DoubleClick to allow additional data to be tracked within Google Analytics

The option I added to enable/disable the new code may not be entirely necessary - I think the DoubleClick code may work with any site, regardless of Google Adwords status. However, it does involve an additional third-party cookie that some site owners may not want. Better safe than sorry. Having that option visible will also inform users that Adwords Remarketing is supported.