Maybe al this method could be added to a separate category, such as `Lockbox+NSSecureCoding`?

Then it could be added as a Lockbox [subspec](http://guides.cocoapods.org/syntax/podspec.html#subspec) and make it [optional](http://guides.cocoapods.org/syntax/podspec.html#default_subspec).

That would probably make it easier to merge while keeping the library simple enough.