README updates.

Just a couple of README fixes:

* I've fixed the syntax error around `sqlx.Open` (which returns both a *DB and an error)
* I've added an example for inserting a struct as although the README indicates it can be done, it doesn't show an example.

I also wouldn't mind adding an example for using `rows.Next()` to scan into a slice of structs, but before I do that: is there any intention to provide a `rows.ScanAll(interface{})` method to do this automatically?