Support optional serialize and unserialize

[Runrioter]

When I am restructuring some existing code that wrote by Java and PHP, I have a problem. Data in store was serialize or unserialize in Java/PHP Object serialization, so I want to make this repo support serialize/unserialize.

Please review it.

[coveralls]

Coverage remained the same at 100.0% when pulling fc77292 on Runrioter:master into 7e4940a on koajs:master.

[DaAwesomeP]

@Runrioter Thank you! I really like this idea. Could you please add tests for serialize and unserialize? It can be something really simple (ex. adding some characters to the send of JSON.stringify() in serialize so that it will fail JSON.parse() unless it goes through unserialize).

[coveralls]

Coverage remained the same at 100.0% when pulling 5d79370 on Runrioter:master into 7e4940a on koajs:master.

[DaAwesomeP]

Thanks!

[Runrioter]

@DaAwesomeP my pleasure

[Runrioter]

@DaAwesomeP Can you release a patch version? I will use it in production.

[DaAwesomeP]

@Runrioter I'm going to wait a day or so to hear from #34. In the meantime, you can specify your fork as a source to NPM.

[DaAwesomeP]

@Runrioter I published v3.1.1.