python script to generate proto3 definitions

[aavaas]

Related to #44 ; C# protoc only takes proto3 definition files and manually converting on each update is a big hassle. With the converter script, proto2 definition files can be kept as it is as upgrading to proto3 is not planned.

The python script creates a new folder "s2clientprotocol_proto3" and writes all the converted .proto files there. I have also added ignore rule for the "s2clientprotocol_proto3" folder in .gitignore file.

The script is not generic and only for current definition files. It does,

1. header change
2. remove optionals
3. add dummy enum element 0, similar to Update to proto3 syntax #44

[NikEyX]

This is very helpful, thanks!