Make MarqueeLabel compatible with auto-layout

This fixes the MarqueeLabel for the auto-layout system. This addresses issue #39.

Here is the code I used to test with as I did not add an example to the existing demo application. 

```objective-c
MarqueeLabel* label = [[MarqueeLabel alloc] initWithFrame:CGRectZero];
[label setTranslatesAutoresizingMaskIntoConstraints:FALSE];
[label setBackgroundColor:[UIColor grayColor]];
[label setText:@"This is a long message. It is used to test if the MarqueeLabel works correctly with auto-layout."];

[[self view] addSubview:label];

NSDictionary* views = NSDictionaryOfVariableBindings(label);

// Layout the marquee label at the top of the view with standard spacing on the left, right, and top. The marquee label is pinned to the top of it's superview.
[[self view] addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-[label]" options:0 metrics:nil views:views]];
[[self view] addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-[label]-|" options:0 metrics:nil views:views]];
```