User enumeration implemented

- TODO: Implement <min balance> and <max balance>
- Updated protocol spec for extended ENUM_USERS
- Client Dispense_EnumUsers implemented
 > Changed padding in Dispense_ShowUser
- Fixed client's ReadLine
 > Was not using the remaining data in the buffer
- Created a common "cokebank.h" for server
 > Moved pseudo-account names into this file
- Implemented ENUM_USERS at server side
 > Copied sendf to the server to improve response times (and memory
   usage)