Documentation in examples recommends using
`tokio 0.2` as dependency, while README.md
recomends `tokio 0.1`.
I've updated comments according to readme.
The "socks" feature has been removed for a while now, the optional
dependency on the "socks" crate commented out.
The code for actually providing the socks feature was, however, still
mostly present, if a bit out of date.
This commit re-adds the socks feature using the tokio-socks (instead of
socks) crate.
Closes#620