Plugins system

[wiktor-obrebski]

Let prepare base for plugins-based application system:

• directly relation between plugins should be forbidden (so it should be forbidden to use first plugin from second plugin by importing his modules or methods)
• plugins should be easily attached/removed from application
• plugins that need some things from each-others should communicate by ServiceLocator or similar pattern
• plugins should have easily maintained information about his relations (what services need to work or what others plugins)

[wiktor-obrebski]

ready, but without docs