small edits on web material


git-svn-id: svn://svn.code.sf.net/p/healpix/code/trunk@437 5e3cd54f-fed5-4059-89be-b68eacfec290
add images


git-svn-id: svn://svn.code.sf.net/p/healpix/code/trunk@438 5e3cd54f-fed5-4059-89be-b68eacfec290
tested on Firefox, Chrome and Safari


git-svn-id: svn://svn.code.sf.net/p/healpix/code/trunk@439 5e3cd54f-fed5-4059-89be-b68eacfec290