Adds PluginLoader to project.

Adds PluginLoader and implements load function, which reads
settings.txt file in the plugin folder and load all classes
described in this file.
Adds plugins_readme.txt which describes how to develop
new plugin.
Adds epmty MessageProcessor interface.
Changes plugin development.

Removes the need in settings.txt. PluginLoader scans the whole dir,
finds all jar files, then scans these files and finds all classes,
that implements MessageProcessor.
Adds processMessage function, which applies each plugin one by one.
Adds calls of pluginLoader to the client and server.
Changes a bit plugin load.

Adds test plugin Censor.

Changes interfaces check of loading class.

Adds list of loaded plugins, printing to the output.

Resolves problem with NullPointerException in server. Wrong place of
pluginLoader init.
Changes name registering in client.

Changes name registering process. Client now can abort the process.
Also it resolves mistake, when client enter duplicate name and
has to retype connect, but does not know about it. Also client now
can type list dusring this process.
Does some interface cosmetics.
Corrects some mistakes in logic.
Corrects some mistakes.

Corrects mistake with interfaces determination in pluginsLoader.

Adds handling connection abortion during name registration process.