Aggregate data from multiple pages of results

The current library fails to return all results for some calls. In particular, the `getRepositories()` method fails to return all repositories for users with a large numbers of repositories (even my account has too many repositories). As a result, other libraries which depend on the result of these methods, such as GitExtensions' Clone Github Repository feature, fail to behave properly.

For API calls that result in a `List<T>`, check the HTTP headers for a `Link` header with a link marked with `rel="next"`. For these cases, include the results of the following pages in the returned list.

I implemented this via a helper extension method `GetList` to aid in conversion of multiple methods that return lists.