Adding `spec:show_matchers` rake task

Sample output (note the paths were truncated)

    have_exactly           rspec-expectations-2.14.4/lib/rspec/matchers.rb:444
    have_form              rspec-html-matchers-0.4.3/lib/rspec-html-matchers.rb:303
    have_multivalue_field  curate/spec/support/matchers/metadata_field_matchers.rb:19
Merge pull request #373 from projecthydra/adding-custom-matcher-lookup

Adding `spec:show_matchers` rake task