API to generate .rs files

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

Example code:

With stable rust-protobuf:

```rust extern crate protobufcodegenpure;

protobufcodegenpure::run(protobufcodegenpure::Args { outdir: "src/protos", input: &["protos/a.proto", "protos/b.proto"], includes: &["protos"], customize: protobufcodegen_pure::Customize { ..Default::default() }, }).expect("protoc"); ```

With rust-protobuf from master:

```rust extern crate protobufcodegenpure;

protobufcodegenpure::Args::new() .out_dir("src/protos") .inputs(&["protos/a.proto", "protos/b.proto"]) .include("protos") .run() .expect("protoc"); ```

And in Cargo.toml:

[build-dependencies] protobuf-codegen-pure = "2.3"

The alternative is to use protoc-rust crate, which relies on protoc command to parse descriptors. Both crates should produce the same result, otherwise please file a bug report.