Add support for the /yo/ api endpoint for yo'ing a specific user

Added support for an api endpoint that was recently added - https://twitter.com/YoAppStatus/status/486231019847028736.

```
Specific user Yo:
 
curl --data "api_token=<token>&username=<username>" http://api.justyo.co/yo/
 
* Rate limited to one Yo per user per minute.
```