Support for issuing EOS tokens + clean up / fixes

EOS Improvements / Feature additions

• Added issue to EOSManager for issuing EOS tokens
• Added customised send_or_issue to EOSManager, ensuring tokens are issued to our own account first, and then sent to the end user
• Refactored parts of EOSManager.send into:
  • get_privkey - Find and decrypt a private key for a given account, optionally filtering by key type
  • validate_amount - Validate and sanitise EOS amounts (4 DP), and optionally check we have enough balance
  • build_tx - Builds an EOS contract transaction, finds private key for sender, signs it, and broadcasts it.
• Fixed bug with EOSMixin.eos_settings - EOSManager previously would not respect RPC node settings from the database
  as it wouldn't load the EOS coin if another token symbol was selected.
• EOSManager.address_valid now takes multiple addresses (instead of just one) as positional arguments
• EOSManager.address_valid_ex throws AccountNotFound instead of a boolean response, allowing you to know which account
  isn't valid, and reduce dependence on if x.address_valid(y) - instead just using try/except.

Other fixes / improvements

• Moved SteemManager's self.asset and self.precision into properties. They were causing slowdowns due to constantly
  querying for assets and their precision every time the manager class was reloaded.
• Added customised send_or_issue (copied from EOSManager) to SteemEngineManager which issues the token to the issuer
  before sending, instead of issuing directly to the user (some problems with issuing directly, including the lack of memos...)
• Improved comments including more PyDoc comment blocks.
• General cleanup, including removing some old code that was commented out