Add support for setting response headers

[marcosnils]

API was taken from the same way nock does it (https://github.com/pgte/nock#specifying-reply-headers)

[tinchogob]

+1! Could you also update the Readme?

[marcosnils]

@tinchogob done.

[xetorthio]

How about we also make reply similar to the request? So it is nestable and offers a nicer API. For example you could do:

mock.get('/foo').set('Authorization', '123456').reply(401).set('WWW-Authenticate', 'Basic realm=foobar');