Support batch types introduced in Cassandra 1.2

If you want to batch counter updates you will receive the following error:

```
(Error) Original Exception: Cql::QueryError: Counter mutations are only allowed in COUNTER batches
```

This commit adds support for counter batched updates and unlogged batches using the `type` key.

```ruby
keyspace.batch({
  column_family_name: 'apps',
  type: 'counter',
  modifications: [
    [:update, set: {downloads: Cassanity.inc(1)}, where: {id: '1'}],
    [:update, set: {downloads: Cassanity.inc(1)}, where: {id: '2'}],
  ],
})
```