First view on the rewrite

WARNING!
This is a pre-release. It's not for the production use, things may change between pre-releases and not everything can work as expected. You've been warned.

• The plugin has been mostly rewritten, allowing for easier addition of version-specific implementations.
• Each supported implementation has its own jar file now.
• Configuration is now identical for every implementation.
• Commands and subcommands are now under one group in the configuration for easier management of permissions.
• Lookup of available commands and subcommands is now even faster than it was.
• Dynamically adding commands to existing groups via a command is now possible on waterfall and velocity implementations.
• The subcommand blocking feature is still missing from velocity implementation.