Support for marshmallow schemas

[ergo]

If I would add support for marshmallow schemas would the project accept the PR?

[leplatrem]

Yes of course :)

The test_validation.py might need some generalization, if you have some ideas please do! Otherwise some dedicated unit tests would be just fine.

Thanks!

[ergo]

Ok, I've started work on this already, https://github.com/ergo/cornice/commit/0701855056b504419de1e900eafa35b875c23c2a and my app returned nice json errors for me with simple and nested schema.

Now its time to write the tests and documentation.

[ergo]

Hi, I've added support for Marshmallow here #475, 100% coverage,
Works on both Marshmallow 2.x (stable version) and 3.x beta branches pass too.

The only thing that is missing right now is documentation effort.

[leplatrem]

Fixed by #475