Allow customization of URL for _trackPageview()

Issue Summary

The attached patch adds a new field in the advanced settings section of the GA configuration form to enter JavaScript to be used as a value for the URL in _trackPageview().

As mentioned in the help text underneath that field, this value will be overridden on certain pages if you use the "Track internal search" and "Track translation sets as one unit" options.

Documentation for using this optional custom url is at http://code.google.com/apis/analytics/docs/gaJS/gaJSApiBasicConfiguratio...

Some discussion of why people use this feature is at
http://stackoverflow.com/questions/4811172/is-it-possible-to-track-hash-...
https://groups.google.com/a/googleproductforums.com/forum/#!category-topic/analytics/discuss-issues-related-to-your-accounts-reports-and-data/5ffbd0U8BNY
https://groups.google.com/a/googleproductforums.com/forum/#!category-topic/analytics/asynchronous-tracking-code-snippet/Xrh8CVB4P-o