Adding kotlin sources to built *-sources.jar

[hebo6]

Does not containing kotlin sources in built *-sources.jar currently, it make using wechaty more diffcult.

[CLAassistant]

All committers have signed the CLA.

[diaozxin007]

LGTM

[huan]

Link to wechaty/PMC#16

[hebo6]

请问有成员帮忙合并这个PR吗

[huan]

I can help you merge this PR because @diaozxin007 has approved it.

[huan]

Thank you very much for your contribution!

You are welcome to join Wechaty Contributor Program

1. Join Wechaty Organization

You've invited Bo He to Wechaty! They'll be receiving an email shortly. They can also visit https://github.com/wechaty to accept the invitation.

I have invited you to join our Wechaty GitHub Organization, please accept it by following the above message. (See also: wechaty/PMC#16)

2. Update Your Wechaty Contributor Profile

1. Please open Contributor Hall of Fame and add yourself to the end of the list, so that other contributors will know you better!
2. Please add yourself to our Website Contributors by creating a PR and refer to this PR link as well.

3. Join The Contributor Only WeChat Room

We also have a WeChat room for contributors only which can discuss Wechaty at a deeper level, you are welcome to join and if you are interested.

Please add @lijiarui wechat: ruirui_0914 and send her this pr link. She will invite you into Wechaty Contributor Room

Cheers!