Put sync.Mutex on top of variables it protects.

This change makes the code more consistent with the rateMu below.

This is a followup to https://github.com/google/go-github/pull/390#discussion_r68863652.

Closes #396.

Change-Id: I0fc8b280911f9171fb4d8e3fa60cf6d53b64a65b

github/github.go

@@ -88,10 +88,8 @@ const (
 
 // A Client manages communication with the GitHub API.
 type Client struct {
-	// HTTP client used to communicate with the API.
-	client *http.Client
-	// clientMu protects the client during calls that modify the CheckRedirect func.
-	clientMu sync.Mutex
+	clientMu sync.Mutex   // clientMu protects the client during calls that modify the CheckRedirect func.
+	client   *http.Client // HTTP client used to communicate with the API.
 
 	// Base URL for API requests.  Defaults to the public GitHub API, but can be
 	// set to a domain endpoint to use with GitHub Enterprise.  BaseURL should