Let Repositories take optional argument for pagination

## Repositories API pagination
GitHub Repositories API returns only 30 repositories.
So if you want to see all of your repositories, you have to request many times with pagination.

To achieve this, I added an optional argument for pagination to Repositories.

## example
```go
user, _ := github.GetUser("octocat")
repos, _ := user.Repositories(1)
...
repos, _ := user.Repositories(2)
...
```