GitHub occasionally changes the IP addresses from which GitHub Pages
websites are served. For users of the service, they must upgrade any A
records to these IP addresses to continue using the service. This patch
provides this data in the `APIMeta` struct as the property `Pages` and
populates from the /meta endpoint.

Release Announcement:
https://developer.github.com/changes/2015-06-11-pages-a-records/

Fixes #212.

Also:
 - add PingHook to RepositoriesService
 - move ListServiceHooks to Client, since hooks are no longer just for
   repos.  Leave behind Repositories.ListServiceHooks for backwards
   compatibility

Fixes #178

okay, so these aren't really that useful buy hey, why not? :)

               MMM.           .MMM
               MMMMMMMMMMMMMMMMMMM
               MMMMMMMMMMMMMMMMMMM      __________
              MMMMMMMMMMMMMMMMMMMMM    |          |
             MMMMMMMMMMMMMMMMMMMMMMM   | Ship It! |
            MMMMMMMMMMMMMMMMMMMMMMMM   |_   ______|
            MMMM::- -:::::::- -::MMMM    |/
             MM~:~   ~:::::~   ~:~MM
        .. MMMMM::. .::::. .::MMMMM ..
              .MM::::: ._. :::::MM.
                 MMMM;:::::;MMMM
          -MM        MMMMMMM
          ^  M+     MMMMMMMMM
              MMMMMMM MM MM MM
                   MM MM MM MM
                   MM MM MM MM
                .~~MM~MM~MM~MM~~.
             ~~~~MM:~MM~~~MM~:MM~~~~
            ~~~~~~==~==~~~==~==~~~~~~
             ~~~~~~==~==~==~==~~~~~~
                 :~==~==~==~==~~

notably, this also updates client.Do() to handle non-JSON response
bodies by passing an io.Writer as the final argument, v.