Integrate optional support for rate-limiting

This decorator seems nice: https://pypi.python.org/pypi/ratelimit/1.1.0

https://stackoverflow.com/questions/40748687/python-api-rate-limiting-how-to-limit-api-calls-globally

This is not bad at all: https://gist.githubusercontent.com/gregburek/1441055/raw/6a5d7075df4c272a533d977037c937d242373abe/rateLimitDecorator.py

Bittrex seems to be capped at 1 request per second: https://github.com/thebotguys/golang-bittrex-api/issues/18