Add fix for issue #37

[krasoffski]

As was noted in the pytest-dev@python.org mailing list,

MarkMapping is only complete enough to fulfill its internal use case.

That is why was decided to duplicate logic of from_keyword class method in our plug-in.
Other solution with checking existence of attributes and parsing pytest version looks even worser personally for me.

[lebovski]

I test that fix locally on both pytest, all works.

[lebovski]

Your fix support of new pytest, version bump?

[ailjushkin]

Reviewed, LGTM