Implement unique_local? method

lib/ipaddress/ipv6.rb

@@ -436,6 +436,20 @@ module IPAddress;
       [self.class.new("fe80::/64")].any? {|i| i.include? self}
     end
 
+    #
+    # Checks if an IPv6 address objects belongs
+    # to a unique-local network RFC4193
+    #
+    # Example:
+    #
+    #   ip = IPAddress "fc00::1"
+    #   ip.unique_local?
+    #     #=> true
+    #
+    def unique_local?
+      [self.class.new("fc00::/7")].any? {|i| i.include? self}
+    end
+
     # 
     # Returns true if the address is a mapped address
     #