generated files (eg nimpb/compiler/nimpb_build) should go in a subfolder

[timotheecour]

eg: nimpb/compiler/nimpb_build => this could go under ./build/bin/nimpb_build

protobuf-all-3.5.1.zip and protobuf-3.5.1 could go under ./build/ ...

.gitignore could add ./build/ so that git status can stay clean (eg after running nimble build_protobuf git status is not clean)

[oskaritimperi]

When I first started to create this package, I had some problems with nimble, srcDir and binaries. Maybe I'll need to take a fresh look.