Implement basic updating

[timvisee]

A basic updating system should be implemented.
This system should check whether new (compatible) versions are available for download, and it must be able to automatically install these updates if wanted.

An universal updater should be available to use on any system. The updated will be hosted on my (@timvisee) personal domain.

If the plugin is running on a Bukkit server, a Bukkit-specific updater must be used that is using update files hosted on Bukkit's website. This is a required constraint for plugins hosted on Bukkit.

Administrators should be notified on login if an update is available.

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The update systems don't have to be feature complete yet, as this will be done at a later time. A rapid release is important for this version. It just has to be able to do the tasks mentioned above.