Thanks for the heads up regarding not committing the API key and shared secret to VCS.
I should have been more clear with the software I'm developing. It's a program run in the terminal by a single user. That user should log in to their account either interactively or by having their credentials in a config file. When logged the program can do calculations on chosen lists or smartlists (for example, total estimated time for each priority level, all tasks that have a combined time estimation of less than or equal to 8, that sort of thing). It should also be able to add tasks from a CSV file the program parses. The program is mainly for my own purposes and specific needs, but I thought why not also share it with the world.
So, my problem is that the users are not centrally stored as they would if the software were running on a web server. My intention is to have a stand-alone program that anyone can download and run locally on their own system. Is this possible for me to do? My only experience with API keys for other services has been developing web based applications run on a server.