Switch to using hash-joiner gem; add Rakefile

hash-joiner contains all the code and tests removed by this commit from
_plugins/joiner.rb and _test/joiner_test.rb. The Rakefile is the proper way to
run all the tests.
Merge pull request #19 from 18F/use-hash-joiner

Switch to using hash-joiner gem; add Rakefile
Pull in upstream data updates
Merge branch 'master' of github.com:18F/hub into stage-notice