Fixed issues with passwords containing unicode special characters.

The main fix is the change to ProtocolHandler's jsonEncode method.  Previously,
it used 'char.IsLetterOrDigit' to see if it needed to be escaped, but some
chars, such as "Ð", count as a letter but still need to be escaped.  The fix is
to check if it's in the right range, rather than using that method.

There's also some changes to those methods for performance and clarity reasons.
Most of this is using a StringBuilder rather than appending to the string.  Not
too important, but it makes things clearer.

MinecraftClient/ConsoleIO.cs

@@ -45,22 +45,18 @@ namespace MinecraftClient
 
         public static string ReadPassword()
         {
-            string password = "";
-            ConsoleKeyInfo k = new ConsoleKeyInfo();
-            while (k.Key != ConsoleKey.Enter)
+            StringBuilder password = new StringBuilder();
+
+            ConsoleKeyInfo k;
+            while ((k = Console.ReadKey(true)).Key != ConsoleKey.Enter)
             {
-                k = Console.ReadKey(true);
                 switch (k.Key)
                 {
-                    case ConsoleKey.Enter:
-                        Console.Write('\n');
-                        return password;
-
                     case ConsoleKey.Backspace:
                         if (password.Length > 0)
                         {
                             Console.Write("\b \b");
-                            password = password.Substring(0, password.Length - 1);
+                            password.Remove(password.Length - 1, 1);
                         }
                         break;
 
@@ -79,12 +75,14 @@ namespace MinecraftClient
                         if (k.KeyChar != 0)
                         {
                             Console.Write('*');
-                            password += k.KeyChar;
+                            password.Append(k.KeyChar);
                         }
                         break;
                 }
             }
-            return password;
+
+            Console.WriteLine();
+            return password.ToString();
         }
 
         /// <summary>

MinecraftClient/Protocol/ProtocolHandler.cs

@@ -285,16 +285,18 @@ namespace MinecraftClient.Protocol
             StringBuilder result = new StringBuilder();
             foreach (char c in text)
             {
-                if (char.IsLetterOrDigit(c))
+                if ((c >= '0' && c <= '9') ||
+                    (c >= 'a' && c <= 'z') ||
+                    (c >= 'A' && c <= 'Z'))
                 {
                     result.Append(c);
                 }
                 else
                 {
-                    result.Append("\\u");
-                    result.Append(((int)c).ToString("x4"));
+                    result.AppendFormat(@"\u{0:x4}", (int)c);
                 }
             }
+
             return result.ToString();
         }
     }