#306 Add command service and set up constructor dependency injection

(work in progress)
- Pass all dependencies via constructor
- Encapsulate command handling more (e.g. split CommandHandler with new CommandMapper)
- Add help command to all base commands at one central point

See AccountsCommand or HelpCommand for an example of the advantages - all necessary functions come from CommandService; objects aren't retrieved through a singleton getInstance() method anymore

src/main/java/fr/xephi/authme/command/executable/email/EmailBaseCommand.java

@@ -0,0 +1,19 @@
+package fr.xephi.authme.command.executable.email;
+
+import fr.xephi.authme.command.CommandService;
+import fr.xephi.authme.command.ExecutableCommand;
+import org.bukkit.command.CommandSender;
+
+import java.util.List;
+
+/**
+ * Base command for /email, showing information about the child commands.
+ */
+public class EmailBaseCommand implements ExecutableCommand {
+
+    @Override
+    public void executeCommand(CommandSender sender, List<String> arguments, CommandService commandService) {
+        // FIXME #306 use getCommandService().getHelpProvider();
+        // FIXME #306 HelpProvider.printHelp()
+    }
+}