Improve mapper definition DSL and simplify Mapper itself

  * Change order in which env must be setup (mappers must be defined at the end)
  * Change mappers DSL to be more inline with relations DSL
  * Introduce `Env#reader` which allows reading relations and loads tuples
    using a mapper
  * Remove horrible `rom.relations.users(mapper: true/false)` interface
  * Change Mapper to simply consist of a header and model and add `Mapper#load`

refs #52
Update benchmark
Make it possible to read a grouped relation

refs #52