Add tests for requests_with_backoff
Implementing requests_with_backoff
Write should use backoffs
write does use backoffs!
Make pingdom collector use requests_with_backoff
docstring for _request_and_assert, response generator inside

Rob requested more clarity for the generic assertion code
Also, the generator is only used in _request_and_assert
Provide full requests API and stop importing as requests

As per Paul's suggestions:
* In requests_with_backoff we use 'from requests import *' so that we provide the full API of requests
* We no longer import as requests - instead we just use it as requests_with_backoff