Allow user to use no cover image

[rodrigogiraoserrao]

Following up on my comment in #16 I think it makes sense to add the possibility of not including a cover image at all.

In my document what I did was \setcoverimage[0]{IST_A_CMYK_POS.pdf} to have an image with 0 width, i.e. no image shows up at all.
IMO, the least one could do is to make a note in the docs that a user can pretend to not include any cover image by doing what I did. The next step would be to have the class not include an image by default and the icing on the cake would be for the class to adjust vertical spacing slightly if no image is included. (Again, IMO I don't even think the spacing is that awkward with no image, just slightly unbalanced.)