Release 0.4.0

• Fixed recursion on dependency resolution.
• Implemented precise unparsing to leverage from existing structures in the given source.
• Implemented refactor.core.Configuration to configure the unparser.
• Renamed refactor.ast.UnparserBase to refactor.ast.BaseUnparser.
• Removed token_map attribute from refactor.ast.BaseUnparser.
• Removed refactor.context.CustomUnparser.
• Changed refactor.core.Action's build method to raise a NotImplementedError. Users now have to override it.