tests: flushdb addition on setUp

* Flushes Redis database before running tests.  (closes #3)

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
general: PEP8 improvements
limiter: optionally callable limit argument

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
docs: PEP257 compatibility

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
docs: integration to single page

Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>
redis: configurable Redis backend

* Passes RATELIMITER_BACKEND_OPTIONS as **kwargs to `Redis` constructor.
  (closes #12) (PR #13)

Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Flask-RateLimiter v0.2.0

Co-authored-by: Tibor Simko <tibor.simko@cern.ch>
Signed-off-by: Jiri Kuncar <jiri.kuncar@cern.ch>