Allow cascade option for delete and other fixes

[guzman-raphael]

• Remove CRUD references. (Fix Replace CRUD references in docs #84)
• Replace unnecessary snake_to_camel_case method with datajoint.utils.to_camel_case.
• Replace schema_virtual_module term for table information. (Fix Replace schema_virtual_module from definition #85)
• Allow cascade argument for delete operations. Defaults to false. When a delete is attempted on a record with a foreign key reference, returns status code of 409 with parsed result like so:

  {
    "error": "IntegrityError",
    "error_msg": "Full exception message goes here, blah, blah, ...",
    "child_schema": "alpha_company",
    "child_table": "Employee"
  }

  Otherwise will return 200 as before when successful or 500 for unhandled exceptions. (Fix Modify delete operation to delete_quick #86)
• Add documentation related to bearer token security issue when deploying on a server. (Fix Document JWT security warning to prevent misuse #83)
• Consolidate/Optimize fixtures for reuse in tests.
• Increased code coverage to 81%.

[Synicix]

Just minor changes