Allow compiling the gateway code to a different go package

[MatthewDolan]

Currently, the expectation is that the generated gateway code (.pb.gw.go) will live in the same package as the generated service code (.pb.go). There is no package import when referencing it.

Add an optional flag that allows package import so that gateway code can be put into another package and import what it needs from the proto code (it appears to only require public functions and types).

The motivation here is that if a project has a client it wants to distributed externally and also wants to use grpc gateway server side, it would be nice to not mandate that the client import grpc gateway.

[MatthewDolan]

This has been resolved.