Add a new crystal client generator (beta)

[wing328]

• create the first version (beta) of the crystal client generator.
• test in travis CI with add_pet and get_pet_by_id

(still a lot to enhance/fix, will open an issue for tracking)

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• If contributing template-only or documentation-only changes which will change sample output, build the project beforehand.
• Run the shell script ./bin/generate-samples.shto update all Petstore samples related to your fix. This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master. These must match the expectations made by your contribution. You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*. For Windows users, please run the script in Git BASH.
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[cvgaviao]

I think you missed its unit tests. at least I was not able to find it when I pulled this change.

[wing328]

@cvgaviao right, I've not yet added the unit tests (java class).

I've only added the integration tests (https://github.com/OpenAPITools/openapi-generator/blob/master/samples/client/petstore/crystal/spec/api/pet_api_spec.cr#L79) to start with.

Welcome PRs to add the unit tests.