Don't print unicode escape sequences for non-ascii chars

Also, we need to use the get_stdout_text_writer because of a
bug in py2.6 in which the `.encoding` attribute is not honored
for `sys.stdout` (http://bugs.python.org/issue4947).

Fixes #1048.

Also manually verified on py2.6:

```
$ aws --version
aws-cli/1.6.6 Python/2.6.8 Darwin/13.4.0
$ aws dynamodb scan --table-name testtable2 --output json
{
    "Count": 1,
    "Items": [
        {
            "foo": {
                "S": "✓"
            }
        }
    ],
    "ScannedCount": 1,
    "ConsumedCapacity": null
}

$ aws dynamodb scan --table-name testtable2 --output text
FOO	✓

$ aws dynamodb scan --table-name testtable2 --output table
-----------------------------------------------
|                    Scan                     |
+-------------------+--------+----------------+
| ConsumedCapacity  | Count  | ScannedCount   |
+-------------------+--------+----------------+
|  None             |  1     |  1             |
+-------------------+--------+----------------+
||                   Items                   ||
|||                   foo                   |||
||+-------------------+---------------------+||
|||  S                |  ✓                  |||
||+-------------------+---------------------+||

```

cc @kyleknap @danielgtaylor