Initial spike on wrapped mappers
Make dumping wrapped tuples work

I'm not exactly sure if a nested array of values is something axiom will
be happy to digest and dispatch individual writes in a joined relation.
We'll see.

The important part here is to provide more info on the attribute level
so that we know how to dump a nested tuple correctly. Its exact format
is something we can easily change later if axiom expects something else.
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
Remove obsolete file
Simplify LoaderBuilder.call
Add Mapper::Attribute::EmbeddedValue
Update devtools and axiom-memory-adapter
Fix mutant config
Update flay config
Add Mapper::Attribute::EmbeddedCollection
Add Mapper#group
Add Relation#group
Update @api tags and add TODO items to Mapper/Header