Implement Issue api endpoint.

[demeesterdev]

implementing the issue api endpoint.

from #133

[ngetchell]

It is my fault that there are so many whitespace issues. @it-praktyk cleaned up the code and I was way too slow to accept the changes that I already reviewed.

You will likely need to pull again from master to solve the merge conflict.

Let me know if you need any help.

[ngetchell]

Also, @tdemeester if there is any code you think we're missing from GitLab-API feel free to bring it in!

[ngetchell]

I will resolve the conflicts and merge this code.

[colombeen]

I could really use the issue API functions... I need to import a big list of issues from a different platform.