Added possibility to add a gradient with multiple colors

Hi, 

I've just added the possibility to add more than two gradient colors. You can use it now like this

```objective-c
PrettyNavigationBar *navBar = (PrettyNavigationBar *)self.navigationBar;
[navBar setTopLineColor:[UIColor colorWithHexValue:@"#616161"]];
        
[navBar setGradientColors:@[[UIColor colorWithHexValue:@"#616161"],
                            [UIColor colorWithHexValue:@"#696969"],
                            [UIColor colorWithHexValue:@"#797979"],
                            [UIColor colorWithHexValue:@"#797979"],
                            [UIColor colorWithHexValue:@"#696969"],
                            [UIColor colorWithHexValue:@"#616161"]]];
        
[navBar setGradientLocations:@[ [NSNumber numberWithFloat:0],
                                [NSNumber numberWithFloat:0.02],
                                [NSNumber numberWithFloat:0.4],
                                [NSNumber numberWithFloat:0.6],
                                [NSNumber numberWithFloat:0.98],
                                [NSNumber numberWithFloat:1]]];
        
[navBar setBottomLineColor:[UIColor colorWithHexValue:@"#616161"]];

```

I think it would be nice for other users. 
Cheers