API to generate .rs files

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

Example code:

``` extern crate protoc_rust;

protocrust::run(protocrust::Args { out_dir: "src/protos", input: &["protos/a.proto", "b.proto"], includes: &["protos"], }).expect("protoc"); ```

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