get started on async functionality

[crcn]

This branch introduces async behavior. Fixes #138.

TODO:

• create runner that converts all existing operation tests to be async

[stalniy]

I'd like to propose design for this:

1. Separate operators and execution logic (Allow to use only specific operators #153).
2. Create AsyncExecutor. It probably will extend StaticExecutor behavior
3. AsyncExecutor waits for promises to resolve before passing resolved results into operator functions

[neetin]

@crcn any timeline when this async functionality feature will be added to master?