fixes #2015 - renamed "reverse_proxy" setting to "assume_secure_protocol".

Also
 * updated [http://piwik.org/faq/how-to-install/#faq_98 How do I configure Piwik when the server is installed behind a proxy?]
 * added [http://piwik.org/faq/troubleshooting/#faq_101 When I use https, graphs on the Piwik dashboard are empty. I also can't export to CSV or view PDF reports.]


git-svn-id: http://dev.piwik.org/svn/trunk@3855 59fd770c-687e-43c8-a1e3-f5a4ff64c105