created photoset grid template + video native

To use:
    {% include photoset.html array=“/path/to/photo/1 /path/to/photo/2
/path/to/photo/3” alts=“alt text 1|alt text 2|alt text 3” layout=“12”%}

 - layout specifies how many images to show in each row.
 - photo array is space delimited.
 - alt text array is delimited by |