Parsing with standard library

With those changes, one can use the standard library's tool to parse xml into Atom or Rss, provided he knows what the format is. `feeds` no longer is only a feed generator, it can also be used as a parsing format !

To unmarshal a raw input:

```go
atomFeed := &AtomFeed{}
err := xml.Unmarshal(rawInputAsByteSlice, atomFeed)
```

My knowledge of atom/rss is weak, so this may not cover all the cases, but at least the samples from the tests are correctly processed.

There still is a little difference between `feeds` marshalling and `MarshalIndent` in the standard library (`encoding/xml`). I have found

```go
atomFeed := ... // Build with feeds library
stdMarshal, _ := xml.MarshalIndent(atomFeed, "", "  ") // that's 2 spaces
```

to give the same result, albeit without the xml header (`<?xml version="1.0" encoding="UTF-8"?>`).