API to generate .rs files using protoc to parse files

This API requires protoc command present in $PATH or explicitly passed to Codegen object (but protoc plugin is not needed).

```rust extern crate protoc_rust;

fn main() { protocrust::Codegen::new() .outdir("src/protos") .inputs(&["protos/a.proto", "protos/b.proto"]) .include("protos") .run() .expect("Running protoc failed."); } ```

and in build.rs:

toml [build-dependencies] protoc-rust = "2"

It is advisable that protoc-rust build-dependency version be the same as protobuf dependency.

The alternative is to use protobuf-codegen-pure crate.

Protoc binary

This crate searches for protoc binary in $PATH by default.

protoc binary can be obtained using protoc-bin-vendored crate and supplied to Codegen object.

This is version 2

In branch 3 of rust-protobuf this functionality is provided by protobuf-codegen crate.