full_domain argument to constructors

[madeleineth]

This lets you log in on domains other than ones ending in ".salesforce.com".

Also drop a redundant test, test_custom_session_success, which runs the same code as test_default_domain_success.

Close https://github.com/simple-salesforce/simple-salesforce/issues/351.

I have tested this locally with tox and against a Salesforce sandbox with a non-salesforce.com domain.

[brno32]

This looks neat!

[madeleineth]

This PR has been sitting here for a while, so I rebased onto master, re-ran the tests, and fixed a resource-deallocation issue elsewhere in the package. I've been running my fork as part of a deployed application for a few months now.