UserAvatar to accept user object. (#337)

* UserAvatar to accept user object.

One of the most important thing with this library is that it does 1 request per function. You have 100% control over how many web requests get made.
UserAvatar breaks that.

UserAvatar now accepts a user, which not only makes you know how many web requests gets made, but might also save on web requests if you have an existing user object.

* Removed dots. Please work, travis :<

* Ohhh... A friend spotted the error!

* `go fmt` and fixed comment... PLS TRAVIS

restapi.go

@@ -262,15 +262,21 @@ func (s *Session) User(userID string) (st *User, err error) {
 	return
 }
 
-// UserAvatar returns an image.Image of a users Avatar.
+// UserAvatar is deprecated. Please use UserAvatarDecode
 // userID    : A user ID or "@me" which is a shortcut of current user ID
 func (s *Session) UserAvatar(userID string) (img image.Image, err error) {
 	u, err := s.User(userID)
 	if err != nil {
 		return
 	}
+	img, err = s.UserAvatarDecode(u)
+	return
+}
 
-	body, err := s.RequestWithBucketID("GET", EndpointUserAvatar(userID, u.Avatar), nil, EndpointUserAvatar("", ""))
+// UserAvatarDecode returns an image.Image of a user's Avatar
+// user : The user which avatar should be retrieved
+func (s *Session) UserAvatarDecode(u *User) (img image.Image, err error) {
+	body, err := s.RequestWithBucketID("GET", EndpointUserAvatar(u.ID, u.Avatar), nil, EndpointUserAvatar("", ""))
 	if err != nil {
 		return
 	}