Allow data set POSTs to be chunked

When sending a large amount of data through to the performance platform
we need to be able to chunk the POST so that we don't time out and get
rejected. The user of the client library will be in charge of picking a
chunk size that makes sense for what they are submitting.

This doesn't wrap POSTs up in a transaction so we rely on the fact that
we have 'immutable' ids. So that if data gets written again it should
overwrite the existing data.
Bump minor version to include new functionality
Allow iterables not just lists

Rather than just lists we should accept anything that is iterable (apart
from strings). This allows us to put lazy generators into a post or
maybe a tuple.

hasattr(data, '__iter__') will not return true for a string, even though
iter() will return for a string. See:

http://stackoverflow.com/questions/1952464/in-python-how-do-i-determine-if-an-object-is-iterable
Merge pull request #31 from alphagov/feature/chunking-posts

Feature/chunking posts