oauth2: add examples for basic/custom HTTP client

- provides a bare and custom context example
demonstrating that http client attributes are
not always passed along.
- adds clarifying note to the oauth2.go NewClient
godoc.
- trim down example_test

Change-Id: Iad6697eed83429c36b9ba0efc43293f4910938fb
Reviewed-on: https://go-review.googlesource.com/36553
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: JBD <jbd@google.com>

oauth2.go

@@ -291,6 +291,10 @@ var HTTPClient internal.ContextKey
 // NewClient creates an *http.Client from a Context and TokenSource.
 // The returned client is not valid beyond the lifetime of the context.
 //
+// Note that if a custom *http.Client is provided via the Context it
+// is used only for token acquisition and is not used to configure the
+// *http.Client returned from NewClient.
+//
 // As a special case, if src is nil, a non-OAuth2 client is returned
 // using the provided context. This exists to support related OAuth2
 // packages.