Pagination support

[appleton]

This PR adds support for GitHub API pagination in two steps:

1. Parse and extract the contents of the Link response header into a hash of pagination links in the base adapter and add a links hash to the payload. I think this is required as there's no access to response headers from the serializer.
2. Next, extract pagination information from the links hash into the meta property for use in the application.

Example usage to fetch two pages:

this.store.query('github-branch', {
  repo: 'some/repo',
  page: 1,
  per_page: 100
}).then((branches) => {
  const next = branches.get('meta.next');
  if (next) {
    this.store.query('github-branch', {
      repo: 'some/repo',
      page: next.page,
      per_page: next.per_page
    })
  }
});