add alt tag to images

[christiespeich]

I added in ALT tags on all of the images for accessibility.

For the links that use background-img I used the title attribute as per this suggestion: http://stackoverflow.com/questions/4216035/css-background-image-alt-attribute

This is my first time contributing to another repo so I hope I am doing it correctly. :-) Thank you for this awesome plugin.

[ANDRBR]

I would change the name of the "alt" variable declared in line 23 and add a line when you set the default image so that the alt of the image is the correct one on the first creation. Like this:

'alt' turns into 'firstAlt'

the if statement end up like this:

if((index === 0 && !defaultImage) || $(this).hasClass('sp-default')){
classes = ' class="sp-current"';
firstLarge = large;
firstThumb = $('img', this)[0].src;
firstAlt = alt;
}

Other than that, seems pretty good to me.