Add redis

Should I check to see if Redis is already enabled?

Fixes GH-163

![selfie-0](http://i.imgur.com/HuEmDsu.png)


/fu

Let's call it "enable" instead of "add".

Let's split this out into three functions:

```drush_termins_pantheon_site_redis_enable_validate()``` - validates input, like we do for other functions
```drush_termins_pantheon_site_redis_enable()``` - gives the user some output, and calls
```terminus_api_site_redis_enable() (in termins.site.api.inc)``` that takes valid input and calls ```terminus_request()```