doc: Fix bugs and update

Rebar is now on GitHub, so links did not work.
Some of the 'ls' lines where mixed up.
"How can I make my new gen_server do something?" was unclear where to
put what. Function say_hello was incomplete.
"How can I test?" was not formatted so unreadable.
Fixed more links.
doc: update part 2
Merge pull request #620 from ArthurClemens/doc_rebar

doc: Fix bugs and update