Reuse connections when making API calls

[stevenjm]

By default, the Python requests module will create a new session object each time it is called. This means that each requests.get call has a separate connection pool and cannot reuse connections created by previous requests.

Here we create a class variable with a requests session, which is used for all instances of the APIClient class.

This does mean that sessions will now be shared between all APIClient objects in a process, even if they use different credentials. I do not believe this is a problem because requests are independently authenticated, but I would appreciate a review from someone more familiar with this code anyway.

[coveralls]

Coverage increased (+0.2%) to 78.899% when pulling 8a80f77 on reuse-connections into 77f25ad on master.

[tzengerink]

Reusing the connection is a nice improvement indeed. There's no real way to add tests for this, is there?

[stevenjm]

We could maybe test to make sure that that the socket module's connect method is only called once per request, but I don't think it's worth it. The code works whether or not it reuses connections, and we already have tests for it working. This is just a performance optimisation.