Implement required attributes

[schwern]

From Moose...

       required => (1|0)
           This marks the attribute as being required. This means a value
           must be supplied during class construction, or the attribute
           must be lazy and have either a default or a builder. Note that
           c<required> does not say anything about the attribute's value,
           which can be "undef".

[tobyink]

I've moved the Mite issue tracker here.

This is now implemented.