Documentation examples of SSL config options; closes #116

No, I don't want `RCurl::curlOptions` used anywhere - it adds complexity for no reason.   You can also just do

```R
GET("https://google.com", cainfo = cert_location2, sslversion = 3)
```

And cainfo is already set to the correct location by httr, so all you actually need is

```R
GET("https://google.com", sslversion = 3)
```

The third example doesn't do what you think it does - if you want config to be persistent, you need to use `set_config()`