Added convenience impl IntoConnectParams for String

[ambiso]

Currently one must convert Strings to &str using .as_str() in order to invoke Connection::connect.
This may be avoided if Strings refer to the implementation of &str for IntoConnectParams

Here's an example before and after this change.

Before:

let conn = Connection::connect(env::var("DATABASE_URL").unwrap().as_str(), 
                               TlsMode::None)

After:

let conn = Connection::connect(env::var("DATABASE_URL").unwrap(), TlsMode::None)

[sfackler]

Seems reasonable, thanks!