API to generate .rs files

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

Example code:

``` extern crate protobufcodegenpure;

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

And in Cargo.toml:

[build-dependencies] protobuf_codegen_pure = "1.5"

The alternative is to use protoc-rust crate, which relies on protoc command to parse descriptors (thus it's more reliable), but it requires protoc command in $PATH.