support for map that holds more than 65535 keys

[kyriosli]

When encoding an key-value map, the encoder treats keys that more than 16 as just less than 65536, which may cause problem when serializing objects with large amount of keys, as shown here. I might issue a pull request as soon as I finished the full compatibility tests if you shall not be available for a fix, but plz be kind enough to push a new release on npm after then, ^_^

[mcollina]

Please send a PR :). I'm low on bandwidth atm.