Support encoding in Postgres
Thanks!
 Thank you! Can I hope on patch-release?
 I'll probably wait a bit as I just released a new one not long ago. Maybe a couple weeks?
 Up to you. But for now I have to use manual installation from github, it's not compatible.
 I just released 2.6.0.
 Great! Thank you!

Hello,

In my case I work with a lot of unicode data. All my databases have a UTF-8 encoding, but I've got errors like this: `UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-5: ordinal not in range(128)`.

I knew about the trick:

``` python
database = PosgresqlDatabase(database, **connection_params)
conn = database.get_conn()
conn.set_client_encoding('UTF8')
```

But if you have multiple workers (or pool) it doesn't help and the error still appears. 

So I provide the pull request.

```
database = PostgresqlDatabase(database, encoding='UTF8', **connection_params)
```

Which solve the problem completely.
