Changes after PR suggestions

Naming changes:

  * JSON tags changed to camel casing.
  * Time attributes abbreviated as "Tm" changed to full "Time".
  * Full "journal" concept changed to "history" (vars, docs, URL).

Other changes:
  * CancelErr changed to string type.
  * Added option to forbid cancellation.
  * README.md rewritten with only an overview.
Add functions to set/delete attributes