Create PrivateMessageService, add some methods

Signed-off-by: Vartan Benohanian <vartanbeno@gmail.com>

reddit.go

@@ -91,17 +91,18 @@ type Client struct {
 	// This is the client's user ID in Reddit's database.
 	redditID string
 
-	Account    *AccountService
-	Collection *CollectionService
-	Comment    *CommentService
-	Emoji      *EmojiService
-	Flair      *FlairService
-	Listings   *ListingsService
-	Moderation *ModerationService
-	Multi      *MultiService
-	Post       *PostService
-	Subreddit  *SubredditService
-	User       *UserService
+	Account        *AccountService
+	Collection     *CollectionService
+	Comment        *CommentService
+	Emoji          *EmojiService
+	Flair          *FlairService
+	Listings       *ListingsService
+	Moderation     *ModerationService
+	Multi          *MultiService
+	Post           *PostService
+	PrivateMessage *PrivateMessageService
+	Subreddit      *SubredditService
+	User           *UserService
 
 	oauth2Transport *oauth2.Transport
 
@@ -147,6 +148,7 @@ func newClient(httpClient *http.Client) *Client {
 	c.Listings = &ListingsService{client: c}
 	c.Moderation = &ModerationService{client: c}
 	c.Multi = &MultiService{client: c}
+	c.PrivateMessage = &PrivateMessageService{client: c}
 	c.Subreddit = &SubredditService{client: c}
 	c.User = &UserService{client: c}