Improved DSL macros in src/dsl/macros.lisp

* src/dsl/macros.lisp: renamed src/dsl/dsl.lisp ->
  src/dsl/macros.lisp; removed individual builder macro; added
  `define-model-class-macro' and
  `define-interface-implementation-macros'
* test/dsl/macros.lisp: new file; unit tests for dsl macros
* test/dsl/package.lisp: new file; package definition for unit tests
  of the dsl module
* jenkins.api.asd (header): updated copyright; added one-line summary
  (system jenkins.api): added module dependency of module dsl on
  module api; adapted to renamed files
Fixed exported symbol *base-url* in src/api/package.lisp

* src/api/package.lisp (package jenkins.api): fixed exported
  symbol *jenkins-base-url* -> *base-url*
Support unimplemented interface implementation in src/api/classes.lisp

* src/api/types.lisp: new file; contains types used by the frontend
  module
* src/api/conditions.lisp: new file; contains conditions used by the
  frontend module
* src/api/classes.lisp (header): updated copyright
  (define-interface-implementations): when encountering an unmapped
  interface implementation during deserialization, insert a marker
  object and signal a condition; deal with marker objects during
  serialization
* jenkins.api.asd (system jenkins.api): added files src/api/types.lisp
  and src/api/conditions.lisp