Feature lazy loading

Let's see what I did in this PR:

- #### Added a configuration option in config.php
If the variable with the name of ``` $lazy_loading_file_number_treshold``` is -1 or 0, then lazy loading is turned off or on. If it is positive number then lazy loading will be turned on if the number of the files in a directory exceeds this value.

- #### Added the Lazy load jQuery plugin
This plugin is responsible for lazy loading. It is available at http://www.appelsiini.net/projects/lazyload.

- #### Added width and height properties for the thumbnails in the CSS files
This is necessary for the plugin to work.


I used a jQuery plugin (http://www.appelsiini.net/projects/lazyload) to lazy-load images.