Implementation of set/get

[romaninsh]

This PR implements:

$m['foo'] = 123;

functionality of the basic Active Record pattern. While the loading of an active record is not supported yet, you can work with the values already.

For reviewer, please check:

• Code looks OK
• Test scripts present
• Well-documented
• No outstanding questions or issues or TODO items

[romaninsh]

I've finished with this PR, so it's fully ready for review and merge. Working on "Persisting" next.

[skondakov]

Why do we need atk core in agile data?

[romaninsh]

because we're using implementation for hooks, containers and initializers:

http://agile-core.readthedocs.io/en/latest/

[skondakov]

Seems good to me, except the 2 comments

[romaninsh]

I'm ticking off the check-lists above, @skondakov