Sm fix date problems on filename vs defined inside post

Hi @beatrizcp87!!! :blush::blush::blush:

Some confusion was happening with the dates of the post (you can ask @crystalspinelli). By default jekyll use the filename to set the published date of a post. A post with filename like

```
_posts/2014-03-18-routes-to-spec-or-not-to-spec-in-a-rails-app.markdown
```

Will by default have the published date as 2014-03-18 (this date define the url and the date showed on the content, see the image).

![routes__to_spec_or_not_to_spec_in_a_rails_app__-_blog_do_time](https://f.cloud.github.com/assets/8128/2471918/ddd38e5c-b02d-11e3-94c6-1e1648363ea0.png)

But if we have a date field defined inside the post frontmatter (see more info here: http://jekyllrb.com/docs/frontmatter/), this date defined there have preference. So If I set inside the post frontmatter a different date, this one will be showed in the url and in the content, differing from the post filename.

I think it makes no sense and causes more confusion than fix problems. My deal here is to fix all posts filenames that have different published date from what was defined inside the post content AND removing at all the use of the date definition inside the post.

I already reviewed all files and also the RSS feed. Seems that I did not forget anything. However a review is desired.

Let me know what you think.

//cc @maurogeorge @crystalspinelli