Add support for youtube videos

Problem/Motivation

I really like the simplicity of using this gallery module for images. So I wanted a very simple way to add links to youtube videos along with the photos in the gallery and have them appear in a Colorbox.

Proposed resolution

I propose introducing the ability to add files in the gallery directory call "You Tube ID".yt (e.g. 3b2YdY8dC7E.yt)
When the gallery is being built, it can recognize these files and generate the appropriate links and javascript to embed the videos using the and the you tube API.

I've attached a patch that does what I've described.

Remaining tasks

The patch needs to be reviewed and tested, and the Readme file need to be updated to describe how to add files.
I've modified the himuesgallery-2-line-description.css file to resize the default image of the video to match the other images in the gallery and put a youtube logo in the top corner of the image. This should be done for the other .css files as well.

User interface changes

None

API changes

None