Fix description of state of GCE.
Merge pull request #81 from rybaktomasz/gh-pages-dev-gce

Fix description of state of GCE.
Add docs about enable_agent, for puppet plugin
Add documentation for salt plugin.
Add include/exclude pacakges in debootstrap docs
Merge pull request #83 from dbalan/expose_debootstrap_include-docs

Add include/exclude pacakges in debootstrap docs
Merge pull request #85 from dbalan/salt-plugin-docs

Add documentation for salt plugin.
Note about aptsrc files being appended with list
Document packages.components setting
Document packages.preferences setting
Add guideline about relying on $PATH