Various fixes to codegen plugin

[stanley-cheung]

Fixes #244
Fixes #245

This PR introduces the following changes:

• Allow source proto files, specified in subdirectories, to be output'ed to the corresponding subdirectories, relative to the base output directory (CommonJS require paths when proto files are in subdirectories #244)
• Allow multiple source proto files, to be generated into its own <proto>_grpc_pb.js files (Support compiling multiple .proto files with a single command #245)
• Fixed an issue where, if any of the request or response type within a service definition points to a type specified in another file (or package), the generated code was not properly alias'ed.
• Added tests for all the above scenarios
• Added tests for closure-style generated code

cc @rwlincoln

[rwlincoln]

Note that the gRPC Node JS plug-in generates files with names of the form <proto>_grpc_pb.js.

I might not have been building the correct branch, but I think the out= configuration option is still required and should be made optional.

[stanley-cheung]

out= should no longer be required. When not specified, filename should be in the form of <proto>_grpc_pb.js

[rwlincoln]

I checked out the correct commit and the out= option is indeed no longer required. The require paths account for the directory hierarchy and it is possible to pass multiple .protos into a single protoc call.

Can the generated files be named <proto>_grpcweb_pb.js or something similar? This would prevent files generated by the gRPC Node.js plug-in from being overwritten.

[stanley-cheung]

Sounds good. Generated files are now named <proto>_grpc_web_pb.js