Make establishing outgoing connections the transport's responsibility.

ClientFactories now have a Dial() method instead of a WrapConn()
method, so that it is possible to write something like meek-client
using the obfs4proxy framework.

This breaks the external interface if anyone is using obfs4proxy as
a library, but the new way of doing things is a trivial modification,
to a single routine that shouldn't have been very large to begin with.