Add Cargo file

Added the Cargo build file. I was able to build the PostgreSQL library with it.
Can we fix these dependencies at the current ref for the master branch? The syntax is

```
git = "https://github.com/sfackler/rust-openssl"
ref = "5fb236128c4c552ace49c2d1f72a8996ee8d05d6"
```

The ref for phf_mac is 09e8d0a3375af00a08a632922e4024337a58fc57.