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!


You could reduce the number of `use` statements by doing something like:
```rust
use std::io::{BufferedReader, IoResult, fs};
use std::io::stdio::stdin_raw;
use std::io::fs::PathExtensions;
```