Proposal: Always generate .pb.go next to original .proto.

[adamryman]

• Update default location of .pb.go to be in the same directory as the .proto definition.
• Remove flag --pbout

This will update truss such that .pb.go files generated are in line with what the rest of the protoc-gen-go users are doing.

See related comments in #201.

• Default truss behavior of putting generated .pb.go files into a different directory than the .proto files that defined them. This confuses protoc-gen-go which truss uses to generate the .pb.go files.
• Proto packages imported in a .proto file used with truss must be imported by their full path relative to $GOPATH/src. This is so that the generated go code will have a proper import path that is buildable.