Improve macros and functions in joxa-core

* and/1, or/1:
  Don't fail with 'invalid-fn-form'-error when being called with one
  argument.

* or/1:
  Fix bug where a wrong result would be returned due to a spurious
  call of joxa-core/and/1 instead of joxa-core/or/1.

* +/1:
  Return zero rather than an empty list when being called without
  arguments.

* -/1:
  Return zero rather than crash with a case clause error when being
  called without arguments.
Merge pull request #53 from KlausTrainer/joxa-core-fixes

Improve macros and functions in joxa-core