feature(extractors/publish_date): Extract publish date from meta tags.

Meta tags:
- meta[name='OriginalPublicationDate']
- meta[property='article:published_time']
- meta[property='rnews:datePublished']
So far checked it on http://www.bbc.com/ and http://www.theguardian.com/

Closes #4
Merge pull request #5 from cronycle/feature/4-publish-date

feature(extractors/publish_date): Extract publish date from meta tags.