[Rprotobuf-yada] added merge

[Romain François]

Hi,

I've added a merge method for protobuf messages of the same type:

 > m1 <- new( tutorial.Person, email = "francoisromain at free.fr" )
 > m2 <- new( tutorial.Person, id = 5 )
 > m3 <- merge( m1, m2 )
 > writeLines( as.character( m1 ) )
email: "francoisromain at free.fr"

 > writeLines( as.character( m2 ) )
id: 5

 > writeLines( as.character( m3 ) )
id: 5
email: "francoisromain at free.fr"

Romain

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Romain Francois
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