Added new /convertallbo2 command

Added the /convertallbo2 command (BO2ConvertFolderCommand.java), which
converts all bo2's in the given folder, and slightly changed the
BO2ConvertCommand so I could use it as superclass for the
BO2ConvertFolderCommand.
Compatible with TerrainControl 2.5 and Bukkit 1.6.4-R0.1 (NOT below)