Implement a basic mapper

Allows metadata mapping from Krikri::Parser objects to ActiveTriples
(and potentially any ActiveModel) objects.

Adds:

  - Krikri::Mapper -- with a mapping registry and top level methods for
    accessing the DSL (#define) and running mappings on a record or
    array of records (#map).
  - Krikri::Parser -- with an interface for accessing values and
    attributes of OriginalRecords. Also implements a basic XMLParser
    and an initial OAI DC parser.
  - Krikri::MappingDSL -- declares properties and child elements in
    a mapping. Gathers various DSL methods in `lib/krikri/mapping_dsl/`.
Add ActiveTriples::Resource#rdf_subject setter