http.js: add FIXME comment

As discussed in the lecture of today, it is wrong
to print the listening message unconditionally.

Rather, we should only print such message in response
to the listening event (see tcp.js)
child.js: describe the purpose of this file
cluster.js: cosmetic changes
cluster.js: more precise comment

While there, make it aesthetically pleasing (according
to my personal view, of course).
cluster.js: tweak previous, even better comment
execute.js: comment on need to check stats type
execute.js: document error handling strategy
execute.js: add obvious comment on exec safety
http_fs.js: add warning re how we handle url path
Merge pull request #11 from bassosimone/patch-2

nodejs: add a number of explanatory and warning comments