Api refactoring

[wintamute]

What do these changes do?

This PR adds a new low-level and high-level API, as discussed in https://github.com/aio-libs/aiodocker/issues/203

Currently only Images and Containers are implemented

Are there changes in behavior for the user?

All existing code is untouched, so there should be no changes in existing code

Related issue number

https://github.com/aio-libs/aiodocker/issues/203

Checklist

• [x] I think the code is well written
• [x] Unit tests for the changes exist
• [x] Documentation reflects the changes
• [x] If you provide code modification, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• [x] Add a new news fragment into the changes folder
  • name it <issue_id>.<type> for example (588.bug)
  • if you don't have an issue_id change it to the pr id after creating the pr
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files."

[barrachri]

@wintamute sorry for the delay, I will try to give you a feedback during the next week!

[wintamute]

@barrachri OK, I was planning on pushing another commit, fixing flake issues and one py3.5 incompatibility, it also took me longer than expected. But any feedback is appreciated

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