Implement resource equality based on identifiers.

This pull request adds two related features. First, it adds resource instance
equality by comparing the classes and identifiers of two resources. If they
share the same class (e.g. `s3.Bucket`) and the same identifiers (e.g.
`name`) then they are considered equal.

```python
>>> b1 = s3.Bucket('boto3')
>>> b2 = s3.Bucket('boto3')
>>> b1 == b2
True
```

Related to equality, this also implements reloading of attributes
after an action has been performed. Imagine the following scenario:

```python
>>> o = s3.Object('boto3', 'test.txt')
>>> o.put(Body='hello')
>>> print(o.last_modified)
datetime(2014, 10, 31, 0, 0, 0)

>>> o.put(Body='hello, world')

>>> print(o.last_modified)
datetime(2014, 10, 31, 0, 0, 20)

>>> o2 = s3.Object('boto3', 'test.txt')
>>> o == o2
True
>>> o.last_modified == o2.last_modified
True
```

Updated documentation and tests to reflect these changes.
Fix typo, split up test
Add a small note about region/aws key/etc [ci skip]
Merge pull request #11 from boto/equality

Implement resource equality based on identifiers.