Allow to pass '&str' or 'String' into 'Pool::new()'
Allow to pass '&str' or 'String' into 'Pool::new()'

This allows to setup a pool like this:

```rust
let user = "someone";
let pass = "password";
let host = "mysql.domain.com";
let port: u16 = "3306";
my:294aa6a5fa0bb7deb774cac4bc63b04d21c12dd5@{}:{}", user, pass, host, port)).unwrap();
```

blackbeam/rust-mysql-simple