LOGBACK-625: create new component that acts as a client for ServerSocketAppender

This request introduces the new SocketRemote component (and its SSL-enabled counterpart).  A SocketRemote can be configured in a Joran configuration files to connect to a remote ServerSocketAppender and log received events according to the local configuration.  As noted in the corresponding JIRA issue, this is functionally equivalent to using a SocketAppender with a SimpleSocketServer or SocketServer, except that direction of connection initiation is reversed.  This should make it easier to create developer tools (such as Logback Beagle) that can receive log events from a remote application that utilizes a ServerSocketAppender.

The changes to the existing code base are quite modest:  added the new component to JoranConfigurator, added a new nested type to DefaultNestedComponentTypes, and added a public modifier to an existing static field.  An existing mock for testing was relocated and modified slightly as well.