Add Gist.from_values

Creating a Gist from a dict of specific values.
Store the file data from Gist.from_values

Create the GistFiles based on the input data.
Store them. Store them all.
Comments and documentation.

Comments, yeah!
Adding tests.
Renaming a test.

Clarity.
Obscuring the client field for the Service.

The Service currently exposes the client with no underscore, which does
mean that in theory you could recursively create an infinite chain:

    client.gitignore.client.gitignore.client.gitignore...

Add underscore to clarify that it should be 'private'.
Create a fixture for the test client.

It's duplicated in all of the client tests at the moment, so is more
useful as a pytest fixture.
Renaming tests.

Renaming for clarity now that the context of the client has been moved
into a pytest fixture instead of being 'inline'.
Extract client fixture for directory.
Swap out client for the fixture.

Swapping out the inlined client for the fixture in conftest.py.
Move the regular expression url to conftest.py.

Move it to conftest.py with the existing client fixture.
Documentation.
Variable and fixture naming.

Lost the context of having the 're_url' defined there, so now it's easier
to follow if it's named 'regex_url'. Similarly, the expected url is now
called 'expected'.
Update test comments formatting.

Consistency.
Removing pytest from gist tests.

It's only used for one test and that test isn't needed.