Update gemfile
Introduce Relation.build

This is just a handy builder method that returns a rom relation that is mapped
by the mapper using `Mapper#call(relation)` interface.

Under the hood the mapper can do *whatever* it's needed to prepare the relation
to be used. It can rename the attributes, perform projection etc.
Add integration spec showing relation mapping
Update spec
Initial work on mapping DSL
Move building of mapper header to Mapping::Definition
Add ability to provide a custom mapper instance
Extract Mapping::Definition into a separate file and refactor
Remove redundant code
Prefer kind_of? over is_a?
Add ability to inject the registry into Mapping.build
Add unit specs for Mapping.build
Add docs for Mapping
Move building default mapper to Definition and add docs
Adjust flay thresholds
Update Gemfile
Merge pull request #60 from rom-rb/relation-mapping-support

Relation mapping support