Better error checking and cobcli output, bugfix

- Consistency and error checking on the credit card payment form has
  been made stricter.
- The payment amount is now always scraped from credit card payment
  screen to resolve a NameError when manually specifying the payment
  amount.
- Confirmation text with an amount is now always shown when making a
  credit payment or account transfer.
- Added dollar sign to credit card payment confirmation text.
Added output samples to README

- All of the documented cobcli commands now include commands with sample
  output.