Add idempotent requests

[snewcomer]

close #443 updated with master. Was hard to resolve all merge conflicts. Thank you @ishikawa

[coveralls]

Coverage increased (+0.01%) to 88.467% when pulling c740949 on sn/indempotent into 5131a67 on master.

[sheharyarn]

How do we configure this? Am I correct in assuming this would be the correct config for this:

config :stripity_stripe,
  ...
  retries: [
    max_attempts: 3,
    base_backoff: 500,
    max_backoff: 2000
  ],
  ...