MapperRegistry => ReaderRegistry
Improve Header interface

  * Make it more hash-like
  * Add `Attribute.coerce`
  * Fetch attributes via method_missing for convenience

refs #53
Expand Header interface with the support for collections

  * Remove method_missing trick since it could be in conflict with
    attribute names and header instance methods
  * Make `ModelBuiler#call` accept a header rather than attribute names
  * Add support for defining multiple group mappings

Refs #52 #53