Add new function "check-update"

[sockenklaus]

This pull request divides the function "mc_update()" into four functions:

• check_update_vanilla(): Checks, if there is an update for minecraft_server.jar
• check_update_craftbukkit(): Checks, if there is an update for craftbukkit.jar
• get_craftbukkit_release_channel(): Help function, that returns the CB download url.
• mc_update(): The actual update function, that invokes the two check-functions to check if an update is necessary.

This gives us the possibility to invoke /etc/init.d/minecraft check-update to check for updates without shutting down the server.

[Ahtenus]

Good one!