Store time of last update as time.Time

This breaks the API, because `Feed.LastUpdate()` now returns a time.Time object instead of an int64.

I thought about not breaking the API, but two things kept me from it:

1. The documentation of `Feed.LastUpdate()` was broken (since https://github.com/jteeuwen/go-pkg-rss/commit/e44514e2cb44f4c315225aafa11ea01ceaadf7c8) anyway, as it spoke of seconds instead of nanoseconds.
2. According to [Sourcegraph](https://sourcegraph.com/github.com/jteeuwen/go-pkg-rss/.GoPackage/github.com/jteeuwen/go-pkg-rss/.def/Feed/LastUpdate/.examples) only [one project](https://github.com/breunigs/frank/blob/master/frank/rss.go#L147) uses `Feed.LastUpdate()`. I'm sending them a PR if this one gets merged and their implementation was broken until recently anyway (because of calling `Feed.CanUpdate()` right before a fetch; see https://github.com/jteeuwen/go-pkg-rss/commit/4840ef15a3d6d5a6d4be9374962ccd20ce2511b5).