Before base64 encoding the _id, UTF-8 encode it

Because we currently bomb out with a Unicode error when we try to base64
encode a unicode class. This encodes the unicode to str using UTF-8
codec.

The new test confirms that we can support non-ascii Unicode characters.

The existing test_data_id shows that the _id does *not* change for
existing, ascii-only ids. So we don't need to flush the buckets.
Fix python format string for 2.6

Python 2.6 doesn't support ``"{}".format(...)``` - it requires ie
```"{0}".format(...)```
Merge pull request #24 from alphagov/utf-encode-ids

Before base64 encoding the _id, UTF-8 encode it