[Fix #296] Adding SearchAsYouType Field

[safwanrahman]

This fixes #296. It should be a small changes. As the field is added at es-dsl version 7.2.0, we should require that version in setup.py
@alexgarel r?

[barseghyanartur]

Thumbs up for the SearchAsYouType addition. This, however, brings us to the branching hell. I still think that one codebase with tiny shims won't hurt at all. Since we can have ES version checks at no cost at all, we could define dummy classes and raise exceptions in case of version/feature conflicts.

[safwanrahman]

@barseghyanartur I can understand that! But this is how the upstream package is maintaining their version and I think its easier for us to align with them!