Remove legacy `help` module's tests

These are no longer necessary. I'm starting a simple clean-up with the
tests so that the complete test suites passes and then I'll move on to
making all modules pass local code-coverage enforcement.

This is related to issue #7.
Refactor the `cli` module and its tests

This refactors the `cli` module so it passes its test suite and has a
clear and well defined interaction with external resources (i.e. in a
way that's easy to stub in a self-contained way).
Refactor and update the downloader's tests

This refactors the downloader module and its tests to pass. It's a
little messy, but it's a working version, which matches what we need to
implement #4. This is also related to #7.
Update the resource-finders structure

This updates the resource-finders' structure to match what's describe in
issue #4. Basic "imgur" link downloading is now implemented.