Add mercurial to the list of packages needed in a dev environment

The dev environment setup procedure fails if mercurial is not installed
on the machine, it cannot download stuff from
http://code.google.com/p/go.net for instance.
write newline after every log message.
Fix #621: add new use/base images section. Include examples.
Merge pull request #1703 from metalivedev/621-usebaseimages

Fix #621: add new use/base images section. Include examples.
fix start doc
Merge pull request #1711 from dotcloud/1709-start_doc-fix

fix start documentation
Merge pull request #1696 from daniel-garcia/1448-add_newline_every_logmessage

Write newline after every log message
Merge pull request #1674 from jbbarth/docs

Add mercurial to the list of packages needed in a dev environment