Add client/resource constructor examples to docs.

This helps to fix some confusion seen in #24 where it wasn't obvious
how to create an ELB client.

New output looks similar to this:

#### class opsworks.Client
A low-level client representing AWS OpsWorks:
```python
import boto3

opsworks = boto3.client('opsworks')
```

cc @jamesls, @kyleknap