Consider supporting groupings by category

[slifty]

The RFC clusters status codes as:

• Informational
• Successful
• Redirection
• Client Error
• Server Error

There is semantic value in these groupings! It would be great if this library would expose the status codes in that structure as well, for users who would prefer the semantic explicitness in their code. For instance this might mean a line like HttpStatus.Successful.NO_CONTENT or HttpStatus.ClientError.FORBIDDEN.

[bickelj]

#97 would be a nice start!