Add support for user's public events

[picnoir]

Hello there,

First of all, thanks for maintaining this library, please accept this humble lovely animated llama as a thank you gift

I would need this endpoint: https://developer.github.com/v3/activity/events/#list-public-events-performed-by-a-user

I'll push a pull request associated to this issue in a few minutes :)