sleep in retry
rename backoff to timeout and do not sleep zero seconds
Merge pull request #21 from rocco66/sleep_in_retry

Add timeout to @retry()
Test @retry() with timeout more precisely
Behave better while timeouting in @retry()

- no timeout to float conversion
- no explicit check for type/value errors
- pass attempt number to timeout callback
- sleep only if timeout is positive
Add with_next()