feat(eventlog): e := EventBegin; e.Done()

```Go
e := log.EventBegin(ctx, "provide")
e.Done()

e := log.EventBegin(ctx, "provide")
e.Close() // implements io.Closer in case you want to register with some lifecycle management system.
```

util/eventlog/example_test.go

@@ -0,0 +1,16 @@
+package eventlog
+
+import "github.com/jbenet/go-ipfs/Godeps/_workspace/src/code.google.com/p/go.net/context"
+
+func ExampleEventLogger() {
+	{
+		log := EventLogger(nil)
+		e := log.EventBegin(context.Background(), "dial")
+		e.Done()
+	}
+	{
+		log := EventLogger(nil)
+		e := log.EventBegin(context.Background(), "dial")
+		_ = e.Close() // implements io.Closer for convenience
+	}
+}

util/eventlog/log.go

@@ -1,6 +1,10 @@
 package eventlog
 
 import (
+	"fmt"
+	"io"
+	"time"
+
 	"github.com/jbenet/go-ipfs/Godeps/_workspace/src/code.google.com/p/go.net/context"
 
 	prelog "github.com/jbenet/go-ipfs/util/prefixlog"
@@ -30,6 +34,15 @@ type EventLogger interface {
 	// Finally the timestamp and package name are added to the accumulator and
 	// the metadata is logged.
 	Event(ctx context.Context, event string, m ...Loggable)
+
+	EventBegin(ctx context.Context, event string, m ...Loggable) DoneCloser
+}
+
+type DoneCloser interface {
+	// Done ends the event
+	Done()
+	// io.Closer is a convenience-alias for Done
+	io.Closer
 }
 
 // Logger retrieves an event logger by name
@@ -53,6 +66,16 @@ func (el *eventLogger) Prefix(fmt string, args ...interface{}) EventLogger {
 	return &eventLogger{system: el.system, PrefixLogger: l}
 }
 
+func (el *eventLogger) EventBegin(ctx context.Context, event string, metadata ...Loggable) DoneCloser {
+	start := time.Now()
+	el.Event(ctx, fmt.Sprintln(event, "Begin"), metadata...)
+	return doneCloserFunc(func() {
+		el.Event(ctx, event, append(metadata, LoggableMap(map[string]interface{}{
+			"duration": time.Now().Sub(start),
+		}))...)
+	})
+}
+
 func (el *eventLogger) Event(ctx context.Context, event string, metadata ...Loggable) {
 
 	// Collect loggables for later logging
@@ -77,3 +100,14 @@ func (el *eventLogger) Event(ctx context.Context, event string, metadata ...Logg
 
 	e.Log() // TODO replace this when leveled-logs have been implemented
 }
+
+type doneCloserFunc func()
+
+func (f doneCloserFunc) Done() {
+	f()
+}
+
+func (f doneCloserFunc) Close() error {
+	f.Done()
+	return nil
+}

util/eventlog/loggable.go

@@ -4,3 +4,9 @@ package eventlog
 type Loggable interface {
 	Loggable() map[string]interface{}
 }
+
+type LoggableMap map[string]interface{}
+
+func (l LoggableMap) Loggable() map[string]interface{} {
+	return l
+}