Update documentation on the hostname setting

The hostname setting was moved from the vagrant plugin to the system
section of the base manifest. These settings were also ordered to make
them easier to read.
Add a note about tarball usage on the manifest

The `"tarball"` setting on the manifest file now specifies if it will
create a new tarball or not, as it will use an existing one regardless
of this setting.
Fix typo on Vagrant section of plugins
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
Add example for relying on path
Move development guidelines into docs
Move taskoverview into docs/
Simply link to docs instead of redirect
Move taskoverview script into package
Fix links to API