Support relay cursor connection for pagination

[natemcmaster]

We currently use very CSharp-ey pagination options (offset, limit). Support other types of pagination, such as Relay's connection cursors.

https://facebook.github.io/relay/graphql/connections.htm

Example:

{
  user {
    id
    name
    friends(first: 10, after: "opaqueCursor") {
      edges {
        cursor
        node {
          id
          name
        }
      }
      pageInfo {
        hasNextPage
      }
    }
  }
}