Add Optional Retry Logic to the Check Methods

[TheConnMan]

Checks should attempt to retry (with exponential backoff for a configurable number of times) posting if they get a 500 class error (400s should not be retried).

Example: Exponential backoff with 3 retries with a starting interval of 250ms (wait 250ms, then 500ms, then 1 sec)

Check post reference:

netuitive-client-python/netuitive/client.py

Line 158 in 76d7b07

def post_check(self, check):