dynamodb module: add backward-compatible support for Python 3.3+

There's one integration test left failed in Python 3, and I am not sure what is the correct way to fix it.

```
FAIL: test_lossy_float_conversion (tests.integration.dynamodb.test_layer2.DynamoDBLayer2Test)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/felix/projects/boto/tests/integration/dynamodb/test_layer2.py", line 470, in test_lossy_float_conversion
    self.assertNotEqual(1.12345678912345, retrieved)
nose.proxy.AssertionError: 1.12345678912345 == 1.12345678912345
```

The problem is, in Python 3, calling str() on a float is not as lossy as Python 2.x. I'm not sure if the LossyFloatDynamizer should always round the float to the same point, or the dynamizer should be kept as-is and we should fix tests to fit Python 3.
I get a Python 2.6/2.7 failure on this one:

```python
======================================================================
ERROR: test_binary_attributes (tests.integration.dynamodb.test_layer1.DynamoDBLayer1Test)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/taylordt/Projects/Amazon/boto/tests/integration/dynamodb/test_layer1.py", line 254, in test_binary_attributes
    'BinaryData': {'B': base64.b64encode(bytes('\x01\x02\x03\x04', 'utf-8')).decode('utf-8')},
TypeError: str() takes at most 1 argument (2 given)
```

Also later there is this, presumably from not using `tests.compat.unittest`:

```python
======================================================================
ERROR: test_binary_attrs (tests.integration.dynamodb.test_layer2.DynamoDBLayer2Test)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/taylordt/Projects/Amazon/boto/tests/integration/dynamodb/test_layer2.py", line 425, in test_binary_attrs
    table = self.create_table(table_name, schema, read_units, write_units)
  File "/Users/taylordt/Projects/Amazon/boto/tests/integration/dynamodb/test_layer2.py", line 61, in create_table
    self.addCleanup(self.dynamodb.delete_table, result)
AttributeError: 'DynamoDBLayer2Test' object has no attribute 'addCleanup'
```