Import from gemnasium

• Import RSpec shared examples, spec, and related Ruby classes from gemnasium project
• Add spec to test shared examples
• Create a Ruby gem
• Move Ruby classes to the namespace of this new gem
• Add a 2-step Dockerfile that exports the JSON schemas, and then builds and installs the gem
• Add a CI config that builds the image; it's very basic, and it'll be improved later on

See gitlab-org/gitlab#333829 (closed) and gemnasium!220 (merged)

Commits are NOT squashed and this is intentional.

Follow-up MRs

• Job to publish the image for a git tag
• Job to run rspec before building the image
• User documentation covering shared examples, helper classes, and image
• Developer documentation explaining how to run tests locally
• Add leading spaces to HERE docs passed to JSON.parse, to align them with the code. See !1 (comment 624534335)
• Rename RSpec examples for Comparable class to make them more descriptive. See !1 (comment 624529471)
• Add Comparable::Report class, and remove cloning from Comparable functions. See gemnasium!207 (comment 615968648)