Better retries

- Allow you to specify which port range you'd like Hamms to listen on by passing `beginning_port` to HammsServer.start

- A way better retries server. You can create test-specific counters by passing in a `key` query parameter or a custom number of tries by passing in a `tries` argument.

Not sure how great the code is, in particular getting a global retry-counter store to Flask was kind of a mess, but it works