Look for DOING_AJAX definition to see if an ajax request is in progress.
Although the AJAX request may not have originated from a bot, from WPeC's perspective we treat it as such to avoid creating a WPeC visitor profile for an AJAX request that doesn't need WPeC functionality.
Merge pull request #1504 from JeffPyeBrook/pr-1503-test-for-non-wpec-ajax-request

Look for DOING_AJAX definition to see if an ajax request is in progress.