Hierarchy fixes, deep get, test coverage

• Ignore directories with no metadata defined
• Give a nice error when .fmf file exists [fix #37]
• Ignore metadata subtrees [fix #43]
• Support for direct deep dictionary value retrieval
• Separate exception for missing tree root [fix #42]
• Move data merging into a separate method [fix #41]
• Ensure that data or parent are provided for Tree
• Test coverage for yaml syntax and finding root
• Do not walk through the whole directory hierarchy
• Example typo, handle yaml parse errors [fix #38]
• Require the same version of the rpm package