Remove rdoc dependency.
Configure RSpec for the test suite.
List challenges.
Retrieve challenges from an API.
Return a successful exit code.
Move test code out of module.
Store credentials in a config file.
Use flags to specify user credentials.
Convert runner to an instance method.

* Creating an instance of `ET::Runner` will make it easier to lazily
  load and cache various settings without having to use class
  variables. I think.
Rename "challenges" command to "list".
The `/sample` directory will be used for testing.
Can download and extract challenges.
Submit solution in current directory.
Can download and extract challenges.
Merge pull request #1 from atsheehan/list_challenges

List challenges