Add rate limiting api and refactor credentials

[thinkjrs]

The current credentials.py and other auth code don't consider rate limiting, which it should. We should implement a means of taking into account:

Header Name	Description
X-RateLimit-Limit	Number of requests allowed per minute on current plan
X-RateLimit-Remaining	Number of requests remaining in current time period (1 minute)
X-RateLimit-Reset	Time at which rate limit will reset

as from the docs

[thinkjrs]

We should use a decorator our in-development query builder class that monitors the response headers and logs to the user.

[thinkjrs]

Closing here in preference of combining w/Romeo's suggestions in #31.