Support for sequences of responses per imposter

[joanlopez]

Similar to #68, but following a different approach:

• The entire response field allows multiple responses, so the user cannot only configure multiple bodies, but can also configure different status codes, etc. Useful, for instance, when mocking non-idempotent endpoints. See the test example.
• It should be backwards compatible, as:
  • it should still be able to read single response (no array) imposters
  • it should behave as it was used to when there's only a single response

In the future, we may want to explore adding different strategies (strategy or responseStrategy field) for how responses are alternated. For now, I guess that sequentially going over all of them is enough.

Closes #165 #31

[kilianc]

LGTM