Feature: hierarchical structure of protofiles

[pustekuchen91]

in this pull request hierarchical structure of proto files will be adressed.
its a follow-up pull request of #12

For example:
companyname/product/proto/x.proto

syntax = "proto3";
package companyname.product.proto;

import "companyname/product/proto/sample_data/y.proto";

message SampleData{
    sample_data.Y y = 1;
}

companyname/product/proto/sample_data/y.proto

syntax = "proto3";
package companyname.product.proto.sample_data;
message Y {
    message Test {
        int32 low = 1;
    }

    Test test = 1;
}