API to generate .rs files

API to generate .rs files to be used e. g. from build.rs.

Example code:

```rust extern crate protoc_rust;

use protoc_rust::Customize;

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

And in Cargo.toml:

[build-dependencies] protoc-rust = "2.0"

Note 1: This API requires protoc command present in $PATH. Although protoc-gen-rust command is not needed.

Note 2: Is advisable that protoc-rust build-dependecy version be the same as protobuf dependency.

The alternative is to use pure-rust .proto parser and code generator.