feat(postLayout): fix github link and add links to twitter and email.

- [x] fix the github link, clicking the author name will redirect you to his github page or to the DefactoSoftware page if no github field is set for the given author.

- [x] Add twitter link and only show it if twitter field is set.
- [x] Add email link and only show it if email field is set.
- [x] Ready to review.

some screenshots:
![screen shot 2014-03-05 at 13 28 09](https://f.cloud.github.com/assets/1220084/2332985/21b56f0a-a462-11e3-8194-f86d7f52168c.png)

![screen shot 2014-03-05 at 13 36 33](https://f.cloud.github.com/assets/1220084/2333020/d19322f0-a462-11e3-98d8-9a57fd18b03c.png)

![screen shot 2014-03-05 at 13 27 54](https://f.cloud.github.com/assets/1220084/2332986/21b9e35a-a462-11e3-911e-82734463a1fb.png)