Added support for setCustomVar (google analytics tracker only)

Conflicts:
	lib/tracker/sfGoogleAnalyticsTracker.class.php
Added an option to insert tracker with only the custom vars in ajax requests

The option is named "insert_custom_vars_if_ajax". When it's set, in ajax
requests, if custom vars have been defined, the tracker will be inserted
but will contain only the push for these vars, this is no a full re-declaration.
You still need to push the _trackPageView or _trackEvent with js, for
example.

This is primary designed like this to work with pjax, because
a) the custom vars may change over the pages loaded in pjax
b) pjax push the _trackPageView at the end of each request

This have been implemented only in the async tracker.
Push the vars and custom vars *before* trackView

Conflicts:
	lib/tracker/sfGoogleAnalyticsTrackerAsynchronous.class.php
	lib/tracker/sfGoogleAnalyticsTrackerGoogle.class.php
Add an example of how it's used.
Add the tracker to the output.