Adding `attributedTruncationTokenString`.

Manually set attributed truncation token string. If specified, `truncationTokenString` and `truncationTokenStringAttributes` will be ignored.

#### At a Glance

With this PR, you can make this: ...[See More](#)
('See More' has attributes but '...' don't)

#### Sample code:

```objc
UIColor *seeMoreColor = [UIColor colorWithRed:0.04f green:0.47f blue:0x78f alpha:1.0f];
NSAttributedString *seeMore = [[NSAttributedString alloc] initWithString:@"See More" attributes:@{NSForegroundColorAttributeName: seeMoreColor}];
NSMutableAttributedString *truncationString = [[NSMutableAttributedString alloc] initWithString:@"..."];
[truncationString appendAttributedString:seeMore];

// set attributed truncation token string
self.summaryLabel.attributedTruncationTokenString = truncationString;
```

Example is not contained in this PR, so you can see full source code here: [devxoul/TTTAttributedLabel:attributed-truncation-token-string]( https://github.com/devxoul/TTTAttributedLabel/tree/attributed-truncation-token-string)

#### Sample screenshot:

![ios simulator screen shot nov 11 2014 12 11 17 pm](https://cloud.githubusercontent.com/assets/931655/4987846/a559f3f8-699c-11e4-8716-d616adcf9ddc.png)