go.net/context: define the Context type, which supports propagating

deadlines, cancellation, and other values across APIs and between
processes.

LGTM=crawshaw
R=rsc, crawshaw
CC=bradfitz, golang-codereviews
https://golang.org/cl/99330045

context/withtimeout_test.go

+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package context_test
+
+import (
+	"fmt"
+	"time"
+
+	"code.google.com/p/go.net/context"
+)
+
+func ExampleWithTimeout() {
+	// Pass a context with a timeout to tell a blocking function that it
+	// should abandon its work after the timeout elapses.
+	ctx, _ := context.WithTimeout(context.Background(), 100*time.Millisecond)
+	select {
+	case <-time.After(200 * time.Millisecond):
+		fmt.Println("overslept")
+	case <-ctx.Done():
+		fmt.Println(ctx.Err()) // prints "context deadline exceeded"
+	}
+	// Output:
+	// context deadline exceeded
+}