Add economic entity interfaces

I decided to add these interfaces, instead of just building upon the User & UserData interface, as support for commerce between clans & users might be something that we want to add in the future in Clans.

Patchwork/src/main/java/me/totalfreedom/economy/EconomicEntity.java

@@ -0,0 +1,8 @@
+package me.totalfreedom.economy;
+
+public interface EconomicEntity
+{
+    EconomicEntityData getEconomicData();
+
+    String getName();
+}

Patchwork/src/main/java/me/totalfreedom/economy/EconomicEntityData.java

@@ -0,0 +1,12 @@
+package me.totalfreedom.economy;
+
+public interface EconomicEntityData
+{
+    long getBalance();
+
+    void addToBalance(final long amount);
+
+    void removeFromBalance(final long amount);
+
+    void setBalance(final long newBalance);
+}