Struct tags for json/cql marshalling/unmarshalling

First off thanks to all developers for making this library. It is really useful. I would request you to generate struct tags for easier json or gocql conversion/usage.

Before using cqlc I used my own structs like the following:

```
type user struct {
    Username string `json:"username" cql:"username"`
}
```

This would be very helpful for using mentioned libraries.
