Added support for Google Analytics campaigns via pretty and relevant SiteTree URLs.

## Preface
This may not be part of the road-map but I thought it was nevertheless Google Analytics related and useful (at least to me!). It sort of sits outside of the more 'general purpose' functionalities in the module at the moment. It is designed to avoid having to hack in redirects all over the place.

## Description
Added support for Google Analytics campaigns via pretty and relevant SiteTree URLs (i.e. as part of the standard site structure).

The GoogleCampaignRedirector is a first-stab implementation of enabling the user to use http://example.com/example to then redirect through to a predefined set of GA campaign information, e.g. http://example.com/actual-page?utm_source=123&utm_name=456[...]. Useful for where a nice url is needed, e.g. print materials being tracked as part of a campaign.