Validate a User. 

A User attempts to authenticate by passing a username and password. 

The User can be of type Client or Staff. The User's Type is returned in the TokenType property of the result. 

If authentication is successful, a Token is generated, and a TAuthenticate is returned with the properties:

• Token = generated Token,
• Status = STATUS_OK = 1,
• StatusMessage = 'Success',
• Identifier = Identifier of User,
• TokenType = Type of user.

If authentication is unsuccessful, no Token is generated, and a TAuthenticate is returned with the properties:

• Token = '',
• Status = STATUS_ERROR = -1,
• StatusMessage = 'Invalid Username/Password Combination.',
• Identifier = BAD_IDENTIFIER = '-1',
• TokenType = ''.

Many functions performing operations with sensitive data, require a Token and return an Authenticate Object. The Token generated in this function should be passed to other functions requiring a Token. 

This function is the same as ValidateClient. 

This function has security level "Public", and can be accessed by anyone without authentication.

[WebMethod(Description = "Validates a login and returns a token")]
public TAuthenticate ValidateUser(string sUsername, string sPassword);