Add a CKAN icon.

For #240.

This doesn't seem to be giving me joy under Linux, but Linux may well
ignore embedded icons. Going to seek test on IRC now.
Transparent icons, larger range of sizes.

Closes #240.
Have our build system actually include the icon.

- Assets moved inside the CKAN directory so they get copied on build.
- `win32icon` property explicitly passed to xbuild. (Thanks nlight!)
- Supression of harmless duplicate resources warnings on repack.
  (All the icons are the same, so we don't care which asembly's icon we use.)
- Closes #240.
Merge pull request #549 from KSP-CKAN/240_icon

Add an icon!