unify `1-getting-started/6_object_wrap` samples to implements the same logics.

[UrielCh]

The 3 example in 1-getting-started/6_object_wrap do not implement the same addon.

• Nan implement value() as a function
• Napi implement value as a field
• node-addon-api implement value() as a function

this PR implement value as a Read and Write field and add a getValue() as a function.

the 2 accessor (set and set) and the getter are now called in the addon.js.

the 3 addon.js files are now the same.

note: the NaN::InstanceAccessor wasn't use in any example before this PR.