use [[bin]] instead of [[executable]]
auto merge of #122 : Benny1992/cargo/master, r=alexcrichton

Tried with [[executable]] and got this:

```bash
FizzBuzz (master*) $ cargo build                                                                                                             mri-2.1.2
unused manifest key: executable.name
unused manifest key: executable.path
       Fresh fizzbuzz v0.0.1 (file:/home/benny/Web/RustApplications/FizzBuzz)
```

I think ```executable``` section should be ```bin```