Make default symbol table conform to the spec

See biscuit-auth/biscuit#94

src/main/java/com/clevercloud/biscuit/token/Biscuit.java

@@ -355,4 +355,25 @@ public String print() {
     public Biscuit copy() throws Error {
         return Biscuit.from_serialize_biscuit(this.serializedBiscuit, this.symbols);
     }
+
+    /**
+     * Default symbols list
+     */
+    static public SymbolTable default_symbol_table() {
+        SymbolTable syms = new SymbolTable();
+        syms.insert("read");
+        syms.insert("write");
+        syms.insert("resource");
+        syms.insert("operation");
+        syms.insert("right");
+        syms.insert("time");
+        syms.insert("role");
+
+        return syms;
+    }
+
+    @Override
+    protected Object clone() throws CloneNotSupportedException {
+        return super.clone();
+    }
 }