Add configuration to support broker SSL

Possibly implements #154, if my assumptions about the implementation are
correct.

broker.go

@@ -1,6 +1,7 @@
 package sarama
 
 import (
+	"crypto/tls"
 	"fmt"
 	"io"
 	"net"
@@ -73,7 +74,12 @@ func (b *Broker) Open(conf *Config) error {
 			KeepAlive: conf.Net.KeepAlive,
 		}
 
-		b.conn, b.connErr = dialer.Dial("tcp", b.addr)
+		if conf.Net.TLS.Enable {
+			b.conn, b.connErr = tls.DialWithDialer(&dialer, "tcp", b.addr, conf.Net.TLS.Config)
+		} else {
+			b.conn, b.connErr = dialer.Dial("tcp", b.addr)
+		}
+
 		if b.connErr != nil {
 			b.conn = nil
 			atomic.StoreInt32(&b.opened, 0)

config.go

@@ -1,6 +1,9 @@
 package sarama
 
-import "time"
+import (
+	"crypto/tls"
+	"time"
+)
 
 // Config is used to pass multiple configuration options to Sarama's constructors.
 type Config struct {
@@ -16,6 +19,11 @@ type Config struct {
 		// KeepAlive specifies the keep-alive period for an active network connection.
 		// If zero, keep-alives are disabled. (default is 0: disabled).
 		KeepAlive time.Duration
+
+		TLS struct {
+			Enable bool        // Whether or not to use TLS when connecting to the broker (defaults to false).
+			Config *tls.Config // The TLS configuration to use for secure connections if enabled (defaults to nil).
+		}
 	}
 
 	// Metadata is the namespace for metadata management properties used by the Client, and shared by the Producer/Consumer.