Example text for images

This is inspired by simpleslides module that defines two types of images:
horizontal and vertical.

The width of the horizontal image should be as wide as possible, without
overflowing the page height.

Vertical images are supposed to be used with some explanation text on the side.
Their width is equal to 0.5\textwidth.

In the next few commits I show how to achieve the same affect without defining
new macros.
Get rid of figure numbers

I don't prefer the figures in presentations to be numbered. So, lets remove the
figure numbering.
Define settings for horizontal and vertical images

The `\definemeasure` bit is from a solution by Wolfgang Schuster on the context
mailing list.