Running multiple cargo commands at the same time leads to errors

```
rustc 1.0.0-nightly (44a287e6e 2015-01-08 17:03:40 -0800)
binary: rustc
commit-hash: 44a287e6eb22ec3c2a687fc156813577464017f7
commit-date: 2015-01-08 17:03:40 -0800
host: x86_64-unknown-linux-gnu
release: 1.0.0-nightly
```

If you start multiple instances of commands like cargo build on the same project the compiler will raise errors.

Probably cargo should lock a project if it is running.
