Add support for package logo's

[basvandijk]

It would be nice if it was possible to associate a logo with a package.

The advantage of a logo compared to just a textual name is that it aids in the instant recognition of a package. Humans are faster at pattern matching graphics than text (TODO: add rigorous scientific proof for this statement ;-) )

Once a logo has been associated with a package it can be used in the hackage page of a package and it can be used in graphical package installers.

Design questions:

1. How to associate a logo with a package?

Probably best to add a logo field to the cabal file, as in: "Logo: mylogo.png". (The logo filepath should be relative to the location of the cabal file.)

2. Should there be restrictions on the logo file?

1. What should be the allowed logo formats? png, svg, etc. It's probably best to restrict them to open formats.

2. Should there be a restriction on the logo width and height? Maybe we need support for multiple sized logo's: a big one to put on the hackage page and a small one to display in package lists. (Maybe we even need support for favicons)