Fixed sendgrid logo and added sendgrid link to sponsors list in footer

The SendGrid logo added by #707 had edges that were not smooth.... Fixed it. Also added their link to our footer. :smile: 

Before:
![before](https://cloud.githubusercontent.com/assets/1220480/5280663/1f4fdeaa-7b1a-11e4-82bc-f71f128b0bcf.png)
 
After
![after](https://cloud.githubusercontent.com/assets/1220480/5280667/23b5d062-7b1a-11e4-9606-e5bfbb3fbbe6.png)