Adds RSS Item Categories as ParsedArticle Tags

[Sonictherocketman]

I noticed that RSS Categories were not being parsed as tags. From my experience they are effectively interchangeable, so I included them as part of ParsedArticle.

I wasn't able to find an example Atom feed (either in the fixtures or cursorily browsing the web) so I didn't include parsing categories for Atom feeds in this change. It also seems like the Atom spec is a bit more involved -- categories in Atom aren't simple strings.

I've added a new test and it does pass. I apologize if my changes aren't paradigmatic; my Obj-C is very rusty.

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btw, I'm using RSParser in a new app I'm developing and it's been a pleasure to use. Keep up the good work!

[brentsimmons]

I wouldn’t be opposed to adding support for categories/tags — but we should support Atom as well as RSS. We should use a single tag datatype that works for both types of feeds. (And for JSON Feed as well. I don’t recall offhand what JSON Feed tags look like.)

[Sonictherocketman]

I can look into this when I get the chance. Finding valid test fixtures in the wild might be tricky, but I'll make something work. IIRC, the parser for JSON feeds already support tags (which is how I found this issue in the first place).