FromStr-esque parsing trait

I think a simple `FromStr`-esque trait would be useful to have for organization and possibly some combinators. I use `FromStr` a lot since its very good for small composable types providing their parsers, and a nom-ified trait for this would be a nice addition.

```rust
pub trait Parsable<O>: Sized {
    type Err;
    fn parse(s: O) -> IResult<Self, O, Self::Err>; 
}
```

