Accessibility improvements to fix errors identified by wave.webaim.org

- Add alt attribute to .lb-image <img> tag
- Add aria-label attribute to .lb-prev and .lb-next <a> tags

src/js/lightbox.js

@@ -99,7 +99,7 @@
     }
 
     var self = this;
-    $('<div id="lightboxOverlay" class="lightboxOverlay"></div><div id="lightbox" class="lightbox"><div class="lb-outerContainer"><div class="lb-container"><img class="lb-image" src="data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==" /><div class="lb-nav"><a class="lb-prev" href="" ></a><a class="lb-next" href="" ></a></div><div class="lb-loader"><a class="lb-cancel"></a></div></div></div><div class="lb-dataContainer"><div class="lb-data"><div class="lb-details"><span class="lb-caption"></span><span class="lb-number"></span></div><div class="lb-closeContainer"><a class="lb-close"></a></div></div></div></div>').appendTo($('body'));
+    $('<div id="lightboxOverlay" class="lightboxOverlay"></div><div id="lightbox" class="lightbox"><div class="lb-outerContainer"><div class="lb-container"><img class="lb-image" alt="Current image" src="data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==" /><div class="lb-nav"><a class="lb-prev" aria-label="previous image" href="" ></a><a class="lb-next" aria-label="next image" href="" ></a></div><div class="lb-loader"><a class="lb-cancel"></a></div></div></div><div class="lb-dataContainer"><div class="lb-data"><div class="lb-details"><span class="lb-caption"></span><span class="lb-number"></span></div><div class="lb-closeContainer"><a class="lb-close"></a></div></div></div></div>').appendTo($('body'));
 
     // Cache jQuery objects
     this.$lightbox       = $('#lightbox');