Ability to list issues per repository with parameters.

Since ```issues.list_repo``` is a private method it shouldn't be used directly. Instead, pushed the functionality to ```issues.list(:user=> 'user', :repo=>'repo' )``` and fixed the params passing for following scenario: 
```github.issues.list( repo: "my_repo", user: "ssmiech", filter: 'all', state: 'closed', labels: "design" )```

Params weren't passed because of destructive ```args.extract_options!``` being called twice.