add Cargo.lock to gitignore file by default

See the [official recommendation](http://doc.crates.io/guide.html#cargotoml-vs-cargolock) 

> * If you're building a library, put Cargo.lock in your .gitignore.
> * If you're building an executable, check Cargo.lock into git.

Rust.gitignore

@@ -9,3 +9,4 @@
 
 # Generated by Cargo
 /target/
+Cargo.lock