Cleaner Bootstrap Gallery Shortcode

Hi,

I create my own gallery shortcode using bootstrap 3 layout.
I want to share it, but also need more cleaner way since I just modify some code I found to create this shortcode.

The code consist of 3 functions:

/* Bootstap Gallery
 * Usage:
 * [btgalleryStart class='for first row only - leave empty for else']
 *		[btgallery-item col="4" src="image url" alt=""]
 *		[btgallery-item col="4" src="image url" alt=""]
 *		[btgallery-item col="4" src="image url" alt=""]
 * [btgalleryEnd]
 */
function lmd_btGalleryTop($atts, $content = null) {
   extract(shortcode_atts(array(
    'bg_class' => 'btgallery'
   ), $atts));
   return '<div class="row btgallery '.$bg_class.'">';
}
add_shortcode("btgalleryStart", "lmd_btGalleryTop");

function lmd_btGalleryBottom() {
   return '</div>';
}
add_shortcode("btgalleryEnd", "lmd_btGalleryBottom");

function lmd_btGalleryItem($atts, $content = null) {
   extract(shortcode_atts(array(
    'col' => 4,
	'src' => '',
	'alt' => ''
   ), $atts));
   return '<div class="col-md-'.$col.'"><img src="'.$src.'" class="img-responsive" /></div>';
}
add_shortcode("btgalleryItem", "lmd_btGalleryItem");

and the usage:

[btgalleryStart]
[btgalleryItem src="https://amrinz.files.wordpress.com/2015/04/gili-trawangan-island.jpg" col=4 alt="just example"]
[btgalleryItem src="https://amrinz.files.wordpres.com/2015/04/gili-trawangan-island.jpg" col=4 alt="just example"]
[btgalleryItem src="https://amrinz.files.wordpress.com/2015/04/gili-trawangan-island.jpg" col=4 alt="just example"]
[btgalleryEnd]

Its working but I think there is another way to join those functions and make it more efficient. Help me to make it look like this:

[btgallery class="myClass" col=2]
  [btgalleryItem src="" alt=""]
  [btgalleryItem src="" alt=""]
[/btgallery]

Thanks for your help