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')
```

The same is done for all clients, service resources, and resource objects. For example, it shows you how to construct an S3 Object as well using its identifiers.

cc @jamesls, @kyleknap