Version 0.3 - libiio v0.21 and new Error handling

• Support for libiio v0.21
• Updated error handling:
  • Support for std::error
  • Implementation changed to use thiserror (from error_chain)
  • Specific types defined for common errors intead of just string descriptions (WrongDataType, BadReturnSize, InvalidIndex, etc)
• New device capabilities:
  • remove_trigger()
  • is_buffer_capable()
• New utility app: riio_stop_all