Fix test fabfile to work on Debian squeeze [#141]

Unlike Ubuntu, it seems that only root is allowed to use
the "apt-key finger" command on Debian.
Refactor require.deb.key() to avoid code duplication [#141]
Fix missing parens in require.deb.key() example [#141]
Fix docstrings in deb module [#141]
Use succeeded/failed result attributes instead of the return code

This is usually more explicit.
Improve the test docs
Upgrade default Redis version to 2.6.14
Upgrade default Node.js version to 0.10.13
Prepare 0.15.0 release