Namespaced classes etc as per issue #19

[amasses]

PR checklist

• [x] An issue/feature request has been created for this PR
• [x] Pull Request title clearly describes the work in the pull request and the Pull Request description provides details about how to validate the work. Missing information here may result in a delayed response.
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Link to issue/feature request: https://github.com/Mastercard/oauth1-signer-ruby/issues/19

Description

This change moves lib/oauth.rb into lib/mastercard/oauth.rb and moves OAuth to Mastercard::OAuth to prevent conflicts at the top level. Fixes #19