Added wrapper option to uuid function

[ben-walters]

Adds wrapper options to .uuid() function to enable stricter validation rules when validating uuid's.

joi.uuid()                  // retains current functionality

joi.uuid({wrapper: true})   // uuid MUST be wrapped by any of the allowed brackets
joi.uuid({wrapper: false})  // uuid MUST NOT be wrapped in any of the allowed brackets
joi.uuid({wrapper: '{'})    // uuid MUST be wrapped by { }
joi.uuid({wrapper: '['})    // uuid MUST be wrapped by [ ]
joi.uuid({wrapper: '('})    // uuid MUST be wrapped by ( )

closes #3081