Create RequestsSessionNetwork

[jmoldow]

This class is the same as the former DefaultNetwork class, except that it accepts an optional session constructor parameter, for clients to specify a session of their choosing rather than having it constructed automatically.

DefaultNetwork is aliased to RequestsSessionNetwork, for backwards-compatibility.

This enables the boxsdk's testing framework to start using the betamax to record real network calls and responses. Although we currently only add one such test, this is a potential alternative to mock-box functional tests in the future.

This also allows for dependents of boxsdk to write betamax tests as well.

[boxcla]

Verified that @jmoldow has signed the CLA. Thanks for the pull request!

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}