add options to disable the check for a working .htaccess file in data and for a working WebDAV server. This are advanced settings that are needed in special situations where our check fail and the user runs into an http timeout.

This should help make OpenSSL configuration on Windows servers easier by allowing the openssl.cnf file to be set directly in the ownCloud config, rather than in SetEnv commands that don't exist and are hard to replicate in IIS.

Remove gravatar and no-avatar functionality, prepare for default avatars even more and reword some stuff

default session_lifetime is 24hrs
recreation of session is triggered at 50% of the session life time

clarify documentation. Fixes https://github.com/owncloud/core/issues/2624 

see #2568

Required by several applications like our pdf viewer

Add support for a reverse proxy that only forwards SSL connections
unencrypted to the web server.

This patch allows to detect the reverse proxy via regular expression for
the remote IP address and conditional overwrite the host name, protocol
and web root.