Add support for remote --first-jar/--second-jar

Example:
java -jar target/SpecialSource-1.3-SNAPSHOT-shaded.jar --first-jar http://assets.minecraft.net/1_4_7/minecraft_server.jar --second-jar http://repo.bukkit.org/content/groups/public/org/bukkit/minecraft-server/1.4.7/minecraft-server-1.4.7.jar