MV implementation

Support for all commandline options except one, see src/mv/mv.rs
Many tests, included a todo list of more tests too, see tests/mv.rs
This is a complete rewrite compared to the current basic mv.

Any feedback very welcome!


```rust
use std::io::{process, fs, FilePermission};
use std::io::pipe::PipeStream;
use std::io::fs::PathExtensions;
```