Make Guid a *string, not a string, in case it is missing

Some Atom feeds, like Github's are missing this value (example: https://github.com/jteeuwen/go-pkg-rss/commits/master.atom).

Yes, the field is technically required, but often missing in practice (Github is not the only Atom provider that fails to include it), and missing values would be more accurately depicted as `nil` than the empty string. (Otherwise, it is impossible to distinguish between a provider that provides an empty Guid and one that provides none at all).


I noticed this issue when working on [Pluto](https://github.com/ChimeraCoder/pluto), because I was assuming that a Guid, if provided, was unique. Databases (both SQL and NoSQL) usually distinguish between `null`/`nil` and the empty string, so it makes sense to reflect that in the Go struct representation as well.



(This probably could apply to some of the other fields, but I'm singling this one out for a pull request because the Guid is the most important field).