Ability to notify via email when hot wallets are low, destination memo support, convert_coins refactoring, plus more

• Add example env file with the most common env options

• Added notify_low_funds, funds_low and last_notified to Coin model

• Added new deposit status mapped - meaning we know the destination details
  but it has not been converted yet, and could still fail

• Add email configuration to settings, with new env vars

• Notify ADMINS if a deposit failed to convert due to low wallet balance

• Refactored convert_coins.py to be two stages (so that low balance email
  can semi-accurately tell you how many deposits are stuck)

  • First stage, deposits are validated, e.g. do they have a valid deposit
    address, or a memo for us to map it to a conversion?
  • Second stage, validated deposits are converted, and conversion object created

• Cache get_token request in SteemEngineLoader to prevent spamming API for
  same token for every single deposit.

• Ignore deposits from market in SteemEngineLoader

• Text after the destination address in a deposit memo should now be used for
  the destination memo

• Clean up logging - commented out un-necessary debug logging, and don't use
  spammy log.exception when we know why the exception happened (e.g. coin daemon dead)

• Probably various small fixes and comment cleanup