Add in tests for a potential `get` method

- Doesn't require a token or smart headers because we're just running http `get` requests for public data
- Should accept json and text/js
- Implements an eminently refactorable 'api_url' idea
Implement a `from_name` data-set method

- just requires a notion of a base/api url (e.g. https://www.performance.service.gov.uk/data/) and a name, e.g. 'my-amazing-data-set'
Change `from_name` to explicitly require an `api_url` arugment

- a config dict with one property seemed a bit silly
Add `from_group_and_type`

- This the method type we really want to support for client reads
- Still clunky but whatever, let's get it in
Add in tests for `get`ting data-sets
Add failing test for dry run true
Correctly pass dry_run option
Add failing test for group and type dry_run
Fix dry_run option for group and type
Add tests for behaviour of make_headers

Prompted by allowing an empty bearer_token - adding a test to be
explicit that this is an allowed case
Merge pull request #6 from alphagov/add_get_method

Add get method