add alternative method for specifying an attribute

Instead of using method missing, use a class method that takes a
attribute name as a symbol
v0.4.0 Added a support for an alternative syntax has_secure <name_of_attribute>
       as per https://github.com/greggroth suggestion (see #2 PL)
Merge branch 'change_to_has_secure_method' of git://github.com/greggroth/has_secure_attribute into greggroth-change_to_has_secure_method