Rework update system

[timvisee]

Rework the whole updating system, and implement it properly.

Work with UpdateRequest and UpdateResponse objects, for creating general update requests and responses. The request object would hold whether updates should be automatically installed, and so on. The response object holds properties like whether an update is available, and whether it is installed.

Use a general updating system using adapters. The existing updaters should be modified to use these adapters. This would adding new updating systems easier later on.

The Bukkit updater should be wrapped to achieve as much as the wanted functionality as possible.

The update manager should provide a directory/file to an updater to download any file updates to. Installing downloaded update files should be generalized, as it doesn't make sense to reimplement this in each new updater.

Even though the updater that is used should be selected automatically, the option should be provided to switch to a different updater. It must be checked whether this is allowed when the plugin is released on Bukkit, as it might not be allowed to use a different updater than the one provided by Bukkit itself.