Rate limiting

[Someguy123]

The bot currently does not rate limit shapeshift or the Block.IO APIs. This could lead to abuse on a large Slack channel, possibly resulting in the bot being banned from one of the APIs.

To be improved:

• Rate limit shapeshift's to one every 2 minutes or so (per user)
• Cache balances, such as via !tipbot check and !tipbot addresses per user, for at least 30 seconds
• Similarly, limit withdrawals to once every 3 minutes or so (per user)