Refactor plugin interface

[tshemsedinov]

Plugin file: impress.<pluginName>.js

1. It exports nothing
2. After require it sets impress.<pluginName>.mixin = function(application) to be optionally called if exists
3. After mixin called it sets application.<pluginName>.init function() to be optionally called if needed
4. On mixin it may set application.on events, e.g. : start, clientConnect, etc.
5. Plugin have a flag mixImpress it indicates do we need mix it to the main application impress

[tshemsedinov]

Merged with https://github.com/metarhia/Impress/issues/511