Adds dimension to Image
Adjusts ExperimentParser to store image dimensions
Adds image dimension to img tag
Changes cellmask rendering back to DOM-Ready

The image tag has fixed width and height now. Therefore the image no more
needs to be fully loaded before we can create the canvas element with the
correct width and height.
Fixes check if cells defined

If you try to access a local variable that is not defined JavaScript raises
an error. To avoid this error we check the type of cells. This returns
always a string -- which is undefined if the variable does not exist.