Edit messages without modifying the JAR file.

Currently there is no way to modify the BungeeCord messages without modifying the JAR file. I know you're still working on better translations, however maybe we can use this as a temporary solution until it is done?

This will add a new `messages.yml` file with the customizable messages in them. Once it is loaded the original ResourceBundle will be replaced by a custom one loaded from the file. The file is saved using `UTF-8` to prevent problems with the special characters in it.

Adds #901 and #1017.

If you don't want to pull this I also have this available [in a plugin](https://github.com/Minecrell/BungeeMessages), but I want to try getting it into BungeeCord directly before publishing this plugin.