Update Forward to accept :to and :through options

Instead of assuming that there's a #strategy method, make it possible to
use an instance variable, or an instance method if :through is not given.
Add Connection abstraction

Didn't really like that strategies had to know about http and the hosts,
so let's abstract that away in a Connection object.
Update README w.r.t. Connection
Don't use options as variable name twice in Forward#forward
Minor cleanup of RoundRobinStrategy
Merge branch 'connection-abstraction'