Add Client Manager to monitor connection state and trigger reconnect (#39)

- Support for exponential backoff on reconnect to be gentle on the server.
- Clean up client by moving metrics and retry strategy to the connection manager.
- Update echo_client to use client manager
- Fix echo client XMPP message matching

Fixes #21
Improvements for #8

backoff_test.go

@@ -0,0 +1,24 @@
+package xmpp_test
+
+import (
+	"testing"
+	"time"
+
+	"gosrc.io/xmpp"
+)
+
+func TestDurationForAttempt_NoJitter(t *testing.T) {
+	b := xmpp.Backoff{Base: 25, NoJitter: true}
+	bInMS := time.Duration(b.Base) * time.Millisecond
+	if b.DurationForAttempt(0) != bInMS {
+		t.Errorf("incorrect default duration for attempt #0 (%d) = %d", b.DurationForAttempt(0)/time.Millisecond, bInMS/time.Millisecond)
+	}
+	var prevDuration, d time.Duration
+	for i := 0; i < 10; i++ {
+		d = b.DurationForAttempt(i)
+		if !(d >= prevDuration) {
+			t.Errorf("duration should be increasing between attempts. #%d (%d) > %d", i, d, prevDuration)
+		}
+		prevDuration = d
+	}
+}