Atom uses rel=alterante to identify links for the source

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
-------------
The Atom RFC in section [4.2.7.2](http://tools.ietf.org/html/rfc4287#section-4.2.7.2) specifies that the `link` element with the attribute `rel=alternate` should be used to point to the article itself.
The previous implementation was seeking for `<link>` elements that had `type=text/html` which caused the retrieval of links for comments:
``` bash
2) FastFeed\Tests\Parser\AtomFirstElementParserTest::testSource with data set #2 ('unawebmaslibre.blogspot.com.xml')
Fail asserting that first element of unawebmaslibre.blogspot.com.xml has source "http://unawebmaslibre.blogspot.com/2013/03/libreoffice-4-desde-ppa.html#comment-form"
```
This PR solves this.