doc: update Github links to halmd-org, few minor edits

The HALMD repositories are now primarily located at Github within the
new organisation "halmd-org"
doc: replace single-quote by apostrophe character

One may argue which character is to be preferred and for which fonts
they are different at all. At least, we should be consistent.
Merge pull request #7 from fhoefling/testing

update Gihub links, minor edits to doc