LOGBACK-819: added support for a logging server that is configurable in logback.xml

This request brings in new SocketServer and SSLSocketServer classes and supporting classes, providing the ability to configure a logging server like any other component in logback.xml.  This allows any application that uses logback-classic to be a receiver for remote logging events.

Touch points with the existing code base are quite minor:  One rule added in classic's JoranConfigurator, a call to register nested component types for the server components in classic's DefaultNestedComponentRules, and the addition of the public modifier to an existing static field that defines the default server port in logback-core.

Included in this request are documentation updates for the new functionality (in logback-site) and some examples (in logback-examples).  The existing documentation for SocketAppender was reworked to include documentation for SSLSocketAppender and the new server components, and the existing Using SSL documentation was expanded.