Fix configuration issues and deprecate time_to_sleep

1. time_to_sleep was deprecated in favor of backoff_strategy
2. Few configurations issues were fixed
3. The pre-defined array of retriable errors was removed.

v0.1.0

Implement backoff strategies PR(#12)
Implement main entry point as a class method PR(#13)

v0.0.3

Fix configurations bug (#4)
Ability to set custom configurations on method call (#1)
More tests (#3)
Update docs and badges