Support querying & setting transport lists on connections.

The list is specified via a magic request property
"X-Android-Transports" and can be queried via the request
property "X-Android-Selected-Transport".

System wide defaults / switches don't need any changes
to okhttp. They can be set as options on the OkHttpClient
used by the platform UrlStreamHandlers.
Merge pull request #204 from narayank/master

Support querying & setting transport lists on connections.