Add tab-completion API. Fixes BUKKIT-2181. Adds BUKKIT-2602

CommandMap contains a method that will auto-complete commands
appropriately. Before the first space, it searches for commands of which
the sender has permission. After the first space, it delegates to the
individual command.

Vanilla commands contain implementations to mimic vanilla
implementation. Exception would be give, that allows for name matching;
a feature we already allowed as part of the command is now supported for
auto-complete as well.

Plugin commands can get a tab completer set to delegate the completion
for. If no tab completer is set, it can check the executor to see if it
implements the tab completion interface. It will also attempt to chain
calls if null gets returned from these interfaces. Plugins also
implement the new TabCompleter interface, to add ease-of-use for plugin
developers, similar to the onCommand() method.

The default command implementation simply searches for player names.

To help facilitate command completion, a utility class was added with
two functions. One checks two strings, to see if the specified string
starts with (ignoring case) the second. The other method uses the first
to selectively copy elements from one collection to another.

src/main/java/org/bukkit/command/CommandMap.java

@@ -62,4 +62,16 @@ public interface CommandMap {
      * @return Command with the specified name or null if a command with that label doesn't exist
      */
     public Command getCommand(String name);
+
+
+    /**
+     * Looks for the requested command and executes an appropriate tab-completer if found. This method will also tab-complete partial commands.
+     *
+     * @param sender The command's sender.
+     * @param cmdLine The entire command string to tab-complete, excluding initial slash.
+     * @return a list of possible tab-completions. This list may be immutable. Will be null if no matching command of which sender has permission.
+     * @throws CommandException Thrown when the tab-completer for the given command fails with an unhandled exception
+     * @throws IllegalArgumentException if either sender or cmdLine are null
+     */
+    public List<String> tabComplete(CommandSender sender, String cmdLine) throws IllegalArgumentException;
 }