documented master/slave connection configuration
documented xss_clean options array
clarified the use of the Opauth callback URL
documented error() method of Validation
Fix a word issue and default values
Merge pull request #586 from hackoh/1.7/document_of_error_method_of_validation

documented error() method of Validation
Add document of error_message of Validation
Merge pull request #588 from hackoh/1.7/document_of_error_message_of_validation_duplicate

Add document of error_message of Validation
本家差分をmerge