Clean up the unleash client and implement the strategies

• unleash-client-php!1 (merged): This merge request includes all the work done by @brianhatchet, complete with a psr compliant caching interface.
• unleash-client-php!2 (merged): Clean up and strategies implementation

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We want to make it Unleash compliant, but for our purposes we can focus on the gitlab version

https://docs.gitlab.com/ee/user/project/operations/feature_flags.html

Specifically, we need to implement the strategies section

https://docs.gitlab.com/ee/user/project/operations/feature_flags.html#feature-flag-strategies