Issue 645. Gists.create doesn't allow specifying visibility bug fixed.

I've added 'visible' parameter to ```Gists.create``` and implementation in mock and real classes implemented ```Gists``. I didn't know which value should be set in different usages of this method, so I kept all clients behavior as it was i.e. set visible=false(according to documentation this parameter is ```false``` by default). I think that developers of issues that were blocked by this now have possibility to set this parameter as they needed. 
Documentation: http://developer.github.com/v3/gists/#create-a-gist