Migrate dataset client into own module

We should be breaking up the functionality of the library into multiple
modules which are then aggregated in the performanceplatform.client
module. This is a common python style to limit the depth at which you
have to import.
Add AdminAPI client

Created an object which encapsulates a connection to the Admin API and
allows all requests required by Backdrop.
Merge pull request #19 from alphagov/add-admin-client

Add admin client