Better plugin system and it's API

[keskad]

I think i must rewrite plugin system and write a good plugins API.

To do:

• "Hooking" module
• Write a layer between Plugins and Subget (something like: Subget.Config.get("subdb", "autosend"), Subget.Plugins.hookup("after-download", self.sendToSubDB))
• API to automaticaly handle all HTTP execeptions (timeout and errors)

[keskad]

I made some progress - the class "SubgetPlugin" which is part of API version 2. To use new API version plugin need to declare that it's written using new API ('API': 2)

Using new API i think that the proxy support from Issue #4 (#4) can be easily implemented.

[keskad]

"Hooking" module - Done
Write a layer between Plugins and Subget (something like: Subget.Config.get("subdb", "autosend"), Subget.Plugins.hookup("after-download", self.sendToSubDB)) - Done
API to automaticaly handle all HTTP execeptions (timeout and errors) - Partialy done