Add finally block to request methods.

By adding finally block to convenient request methods, user can extract common code in both success and failure blocks into finally block, so that there will be less duplicated code.
This pull request keeps the request methods without finally blocks for back compatibility.

So instead of writing this:
```objc
    [SomeIndicator startAnimating];
    [[AFHTTPRequestOperationManager manager] POST:someURLString parameter:parameter
        success:^(AFHTTPRequestOperation *operation, id responseObject) {
            [SomeIndicator stopAnimating];
        } failure:^(AFHTTPRequestOperation *operation, NSError *error) {
            [SomeIndicator stopAnimating];
        }];
```

Now user can write like this:
```objc
    [SomeIndicator startAnimating];
    [[AFHTTPRequestOperationManager manager] POST:someURLString parameter:parameter
        success:^(AFHTTPRequestOperation *operation, id responseObject) {

        } failure:^(AFHTTPRequestOperation *operation, NSError *error) {

        } finally:^(AFHTTPRequestOperation *operation) {
            [SomeIndicator stopAnimating];
        }];
```