add support for setting response headers on Problem subclasses

[edaniszewski]

e.g. if we have a subclass of error which we want to include a response header when it is raised and an error returned, we should be able to do so by defining it on the error class subtype.

class Custom(Problem):

    headers = {
        'some-header': 'foobar'
    }

we may want to also have a way to set more dynamic headers, but I think that is a separate future issue.