Patch cqlengine: add async query functions

[CharAct3]

Add async functions for cqlengine:

# async functions for Model
Model.async_get()
Model.async_all()
Model.async_create()
obj.async_update()
obj.async_save()
obj.async_delete()

# async functions for QuerySet
Model.objects.async_get()
Model.objects.async_all()
Model.objects.async_create()
Model.objects(id=obj_id).async_update()
# also: Model.objects.filter().async_all()
#       Model.objects.if_not_exists().async_create()

WIP:

• Add more unittests.
• Update docs.

ref: #8.

[FedirAlifirenko]

hi @CharAct3
Please rebase onto master to include .travis.yml changes if you are still interesting in passing CI steps.

[CharAct3]

Hi @FedirAlifirenko
It seems the CI is still broken. The newest commit in master branch didn't pass CI steps: https://github.com/aio-libs/aiocassandra/runs/119188360

[hellysmile]

Hey @CharAct3, thats looks really awesome!
Can we have a list what is not ported yet?

[FedirAlifirenko]

Hi @CharAct3
It looks like the table system.size_estimates wasn't filled with expected data before test statement was executed. Your changes aren't the cause.
Hi @hellysmile
What do you think about adding of separate table with testdata for the tests purposes? Bcs all tests are passing when executing with tox at local env.