Tree.node, fmf identifier and minor improvements

• Add git to BuildRequires (now required by tests)
• Use coveralls to report coverage results
• Adjust Tree.node() implementation, extend tests
• Get metadata for unique id [fix #67]
• Use the fedora-all target in packit config (#72)
• Define the full fmf identifier [fix #52]
• A minor cleanup of the concept documentation
• Always ignore special system directories
• Filesystem root and inaccessible dirs [fix #70]
• Rename the 'testsets' directory to 'plans'