add pagination support to Activity.ListWatchers

this is a breaking change, but there's no way around that, since the opt
param was left off initially.
renaming RepositorySubscription to Subscription

I changed my mind.  This really is terribly verbose, and looking at the
design of the API, it seems unlikely GitHub would change the
subscriptions endpoint to include anything other that repositories.  And
if they do...  well, we'll deal with it.
implement remaining Activity.Star* methods