This commit adds UUID support to Vault, allowing plugins to bypass the
OfflinePlayer methods which result in Bukkit trying to resolve a player
to associate with the OfflinePlayer (via the server playercache and if
that player doesn't exist via Mojang.)

This is incredibly useful for any plugin which wants to have an Economy
account that isn't associated with a player. This includes Towny,
Factions, Shops plugins and others.

Most importantly: having UUID methods will give these plugins an avenue
to update from using the String accountName methods deprecated since
Vault 1.4, which doesn't result in slow OfflinePlayer creation.

AbstractEconomy has been updated so that the various Economy plugins
supported internally by Vault will have support for the new methods in
the same manner as when the OfflinePlayer methods were added.

Small javadoc typos have also been fixed up (extra {'s, an additional
{@link, etc.)

These methods are meant for players, non-players and anything with a
UUID.

To match the PR I have opened at the Vault repo, which has had the
native economy plugin support removed, the VaultAPI plugin no longer
requires the AbstractEconomy class.

Removal means that this Pull Request no longer calls
Bukkit.getOfflinePlayer(uuid), making this much safer.

…rt economy providers that are already able to hook into Vault.

Added MultiEconomy which considers an economy as a currency, granting a multi-currency abstraction.
Provided wrappers that help registering/hooking all three types of layers into Vault.