Make API a loadable Plugin

The API should be able to be loaded as a Plugin, so that Bukkit takes responsibility for loading it. An API should never be included in Plugins, nor should each Plugin add a manfiest, just to load an API, because this would be complicated. This PR will fix each problems, because now Admins can just download current Version of MethodCommand and Developers can just link to the Plugin, like Vault-dependent Plugins do.

PS: Will you ever upload MethodCommand to BukkitDev? Would be quite handy.