Documentation examples of SSL config options; closes #116

Does this model what you want?  I don't want to burden Travis with another build.

I'm not happy with my phrase below, "Common configurations can be specified directly, without config()".  Is there a link somewhere that lists the options?  Is it http://curl.haxx.se/libcurl/c/curl_easy_setopt.html (excluding the prefix `CURLOPT_` from each one)?  I'm having trouble following the path of the configuration settings through `http--request.r` and `perform.R`. 

```
#Use HTTP without configuration settings.
result_1 <-  GET(url = "http://rstudio.com")
result_1$status_code

#Use HTTPS with the SSL certificates provided in httr.
curl_options <- config(sslversion = 3)

result_2 <- GET(url = "https://google.com", config = curl_options)
result_2$status_code

#Common configurations can be specified directly, without config().
result_3 <- GET("https://google.com", sslversion = 3)

#Similiar to the two before, but applies to all subsequent calls.
set_config(sslversion = 3)

result_4a <- GET(url = "https://google.com")
result_4a$status_code

result_4b <- GET(url = "https://google.com")
result_4b$status_code

#If you need to specify a different cert file, config() can be used.
#   Assign cert_location the path to your file. 
#   By default, httr uses the cert file included in the package.
cert_location <- system.file("cacert.pem", package = "httr")
curl_options <- config(sslversion = 3, cainfo = cert_location)
result5 <- GET("https://google.com", config = curl_options)
result5$status_code
```