Added timeout functionality to plugin

- Added 'timeout' option: If set to 'true' it will use default 12000 millisecond or you can enter the number of milliseconds for the timeout.

examples: 
  var imgLoad = imagesLoaded($('.images'),{timeout:true}); // uses default 12 seconds
  var imgLoad = imagesLoaded($('.images'),{timeout:1500}); // overwrite default to 15 seconds