WIP initial stab at join/wrap/project in relation mapping
Add unit specs for Mapper#{join, project}
Remove dumper and loader classes (no longer needed because we have morpher)

Also:

  * Make model an optional arg in Mapper.build and default to OpenStruct
  * Make model and morpher node type part of mapper instance state
  * Equalize mapper on header and options
Rename Loader => LoaderBuilder
Add Guardfile
Add unit specs for Header#{join,project}
Add unit spec for Mapper::Attribute#to_ast
Add unit specs for Header#to_ast