Implemented handling of all possible relay responses

[kriptonix]

• Implemented relay responses. Each response has it's own class (RelayResponseOk, RelayResponseEvent, RelayResponseClosed, RelayResponseEose, RelayResponseNotice). For response type using enum (RelayResponseEnum).
• Implemented response validation and added ERROR as response type in RelayResponseEnum to handle edge cases when response is not valid. When response is not valid we are using RelayResponseNotice to notify user with human-readable error message
• EXTRA: Added URL verification in Relay class

[Sebastix]

@kriptonix Thx for the work so far on this :) I don't know how to push changes to this PR, but I extended your work and can be reviewed here #60