Documentation examples of SSL config options; closes #116

As of 0.3, you never _have_ to use config - you can always use named arguments to `GET()` etc. However, it might still be useful if you want to bundle a set of options together.

Most options are passed directly through to RCurl (I think the only important exception is token, used for OAuth2 config). You can get a complete list of options from `RCurl::listCurlOptions()` - it might be worthwhile to expose this from httr, along with a translation guide to the libcurl options.  It looks like the curl page has changed recently so now each option gets its own webpage - maybe I should add a helper to httr that lets you jump directly to that page.