Manifest format and usability improvements

[russellhaering]

Manifest format and usability changes

• Revamped the manifest format to address #23
• Stricter validation of the manifest format
• Improved error messages when the manifest cannot be found
• Honor the 'target' image configuration
• Update the example manifest

[robszumski]

Looks good. This would have helped me quickly get to the bottom of referencing a path to a config file that didn't exist.