[ADDED] CustomDialer interface/option setter

[kozlovic]

We recently added a Dialer option, however, we made it a *net.Dialer,
which is very restrictive (since *net.Dialer is a struct, not an
interface).
One could imagine creating a Dialer that would create pipes between
the client and an embedded server. Since Dial() returns a net.Conn,
which is an interface, everything would work the same.

Resolves #329

[coveralls]

Changes Unknown when pulling a2b049b on add_custom_dialer into ** on master**.

[kozlovic]

This is also quite useful. For instance, one could create a custom dialer to return a connection that can trace Read (and/or Write), or introduce some delay, errors, etc... that could be useful in tests.

Here is an example of a CustomDialer to create a traceable connection:

type MyDialer struct{}

type TracedConn struct {
	net.Conn
}

func (tc *TracedConn) Read(b []byte) (n int, err error) {
	n, err = tc.Conn.Read(b)
	if err != nil {
		fmt.Printf("@@ conn err: %v\n", err)
	} else {
		fmt.Printf("@@ conn read: %s\n", b[:n])
	}
	return n, err
}

func (md *MyDialer) Dial(network, address string) (net.Conn, error) {
	d := &net.Dialer{}
	c, err := d.Dial(network, address)
	if err != nil {
		return nil, err
	}
	return &TracedConn{Conn: c}, nil
}

func main() {
	myDialer := &MyDialer{}
	nc, err := nats.Connect(*urls, nats.SetCustomDialer(myDialer))
       (...)
}

[derekcollison]

LGTM