Vault currency integration

This adds support for virtual currencies through the plugin Vault. A Vault currency can be specified in the config as follows:
```
plugin-currencies:
  d: Vault
  $: Vault
```
where d and $ become characters usable on signs for Vault currency.

I subclassed ShopMaterial to ShopCurrency, which could be subclassed again if support were to be provided for other plugins with multi currencies. e.g. `d: craftconomy:Doubloon` Or something like that.

I know this plugin was designed for material currencies, but it handles multiple currency types already and so plugin currencies fit in pretty well.

Also I had to update some of the libs because I couldn't find downloads for jars of those versions.