[book] some more documentation and cleanups
* merged existing facets documentation with the one I've just written and
put almost everything within the code
* added a note about __depends__ and __recommends__ and about the recommends
semantic (extracted from a post on the ml)
* added a note about write security checking (extracted from a post on the ml)
* fixed some dumb sphinx errors
\.svn
^build$
^dist$
\.pyc$
\.pyo$
\.bak$
\.old$
\~$
\#.*?\#$
\.swp$
^doc/book/en/apidoc$