Added isEmpty() to Duration

Signed-off-by: Tom Wright <tom@inflatablecookie.com>
Added duration handling to format helper

Signed-off-by: Tom Wright <tom@inflatablecookie.com>
Added isSigned to schema traits

Signed-off-by: Tom Wright <tom@inflatablecookie.com>
Set default duration rendering to one unit

Signed-off-by: Tom Wright <tom@inflatablecookie.com>
Set Duration field type to be unsigned by default

Signed-off-by: Tom Wright <tom@inflatablecookie.com>