Merge pull request #1 from MongeziNombewu/features/improvements-for-everegg

[single-kzm_book] Added uses of external referrences json services.
Merge pull request #2 from MongeziNombewu/features/improvements-for-everegg

[single-kzm_book] Fixed issue of null value being used in in_array() met...
Auto hide custom fields
updated mxpress.php
Fixes to calling JSON service
Changed json url to prod
[admin] Added billing settings section.
[admin_options] Added settings fields for MxitVendorID and json service url for external references.
[functions_admin] Added mxpress_billing_section_text() method to render billing section.
[single-kzm_book] Used mxpress optins for serviceURL and item Type OID.
[admin_options] Added virtualItemTypeOID to mxpress options.