Sponsors page + PullReview mention + Sendgrid logo

Based on our discussion @andrew, I have

* extracted sponsors details into a dedicated page and added to it:
  * sendgrid logo (from https://sendgrid.com/resources/logo_brand and refit to 200x75)
  * PullReview details and instructions
* added a sponsor link into the footer
* replaced the sponsor section into the about page with a link to the sponsors page.

The page looks like
![sponsors-page](https://cloud.githubusercontent.com/assets/1059797/5251872/1d71e1a0-7999-11e4-852d-ff71ceb9fac4.png)

Does it correspond to what you had in mind?

For the pullreview gift, I think to show it only if you achieve the goal. What do you think?