always_has should support setting a default value

[kaiwren]

for example
class Foo
always_has :tags, :default => []
end

[ardbytes]

I think a small change can simplify the implementation:

class Foo
  always_has :tags => []
end

If the entry is a hash we use it's key as the attribute name and corresponding value as the default value. If it's just a symbol then we use it as only an attribute name.