Add field and get/setters for "followRedirects"

This should go with a corresponding change to NetJavaImpl so that this field will determine whether redirects will be followed. Useful when it's necessary to get information from a redirect page or when reaching the redirect page is enough.
Added exception and doc comments

Added an exception to setFollowRedirects so that GWT-backenders don't accidentally set it to false (which will not work). Also added doc comments so that these functions are added to the API.
Merge pull request #2054 from WillemWillemWillem/patch-1

Add field and get/setters for "followRedirects"