Refactor Recruiter to use JobPoster.

Refactor specs Recruiter class to transfer the #post_job functionality to the new JobPoster class, which wraps around Recruiter.
Wrap JobPoster around Recruiter implicitly.

Instead of having JobPoster as a stand-alone class, make it a child of SimpleDelegator and use wrapped Recruiters directly instead.

Add the JobPoster#assign_role_to class method, effectively an alias for JobPoster#new but closer to the domain language.
Move JobPoster from recruiters.rb to jobs.rb.

Move the JobPoster class from recruiters.rb to jobs.rb, as the class is more specific to the "those who deal with jobs" actor role than the "recruiter" role.
Refactor PostedJob as a child of SimpleDelegator.

Refactor PostedJob to inherit from SimpleDelegator in order to reduce code duplication between it and UnpostedJob and to make the relationship between the two more explicit.
Merge branch 'jobposter'.

This breaks the job-posting functionality into a wrapper class applied to Recruiter.

This also makes PostedJob a child of SimpleDelegator.
Replaced "recruiters" with "employers".

Per updates to the requirements, take every reference of "recruiter" and change it to "employer".