Implement readonly tag for primary and attr

[jrdnull]

If the readonly tag is present the field will be skipped when using the
Unmarshal methods.
Resolves #102

Opened against and based off of the feature/embeded-structs branch to avoid conflicts, can re-open against master once that is merged, just wanted to get this discussed/reviewed to see if this is something the project is interested in supporting.

The main use case for this is to easily support PATCH requests without resorting to pointers in my models or copying and resetting values.

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[jrdnull]

I signed it!

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CLAs look good, thanks!

[3bb]

Any update ?