Introduce auditable.audit_sql for fetching next audit insert query

[amkisko]

Done:

• README with short description of feature
• auditor got audit_sql method
• test coverage for audit_sql method

Background:

• audited gem is fully relying on logic happening in callbacks and models
• if we skip callbacks, we skip audits
• would be nice to have audits even for large custom operations
• would be nice to collect queries and run them whenever it is required