Added RFC for multiple trait implementations for a single impl block

This pull request will allow the following syntax:

```rust
impl MyFirstTrait + MySecondTrait for MyStruct { //the RFC will allow this 
    //... method impls here
}
```

As said in the [reddit thread](http://www.reddit.com/r/rust/comments/25hsft/potential_rfc_for_multiple_trait_implementations/), this RFC wold probably be a post 1.0 thing, but I wanted to get conversation started for this.

Also I would not be able to implement this, I would love to be able to, but the knowledge is not in my brain!

Any feedback? Would love to read more.