Add implicit reverse references.

This pull request adds support for resources to reference their parents,
for example:

```python
obj = s3.Object('boto3', 'sun.jpg')
print(obj.bucket.name)
```

This works by using the `subResources` feature of the resource JSON
description format. Since `Object` is listed as a child of `Bucket`
in the bucket's sub-resources, and we know the identifier mapping
from and object's `bucket_name` to a bucket's `name`, we can create
the reverse reference just like if an explicit `hasOne` was defined
in the JSON.

Additionally this fixes the SQS resource JSON to no longer explicitly
list the Message hasOne Queue relation, since the Queue has a Message
in its sub-resources.

Adds a test for the resource model as well as modifies the references
test to ensure both normal and reverse references become attributes
on the instantiated resource.
Fix Python 2.6 problem
Fix test hasattr
Merge pull request #9 from boto/references

Add implicit reverse references. Fixes #9.