Feature/sdk module and auth

[easingthemes]

Pull Request Checklist:

• [x] I have signed the Adobe Open Source CLA.
• [x] I have read the CONTRIBUTING document.
• [x] My code follows the code style of this project.
• [x] My changes include automated tests with the expected coverage.
• [x] All new and existing tests pass.
• [x] I have updated the documentation accordingly.

[bdelacretaz]

In the meantime @gknob notes that we might (or rather should) base our clients on https://github.com/adobe/aio-sdk-core which already defines things like config, errors, logging etc. and maybe define our own aem-clients-core extending from that and reusing the https://github.com/adobe/generator-aio-app/tree/master/generators patterns.

@easingthemes based on your work on https://github.com/adobe/aio-lib-customer-profile do you see such synergies? If yes I suggest that we discuss this further, or if you can quickly "morph" this PR to take those comments into account that might also be a good starting point.

[easingthemes]

@bdelacretaz I've checked a bit deeper if we can use https://github.com/adobe/aio-sdk-core, and unfortunately every part of it (Config, Errors, Logger) is written for NodeJs only. For now our SDK support both NodeJS and Browser, if we decide to split it, than we can use aio-sdk-core for NodeJS part. Browser part should always stays with minimal dependencies (currently none)