Specs should run in a clean environment

[mikfreedman]

Unless explicitly specified with the with_temp_calendar_assistant_home helper, specs can (and sometimes do) access the user's local configuration in ~/.calendar-assistant*.

This is probably not ideal because:

• Tests (especially unit tests) probably shouldn't access local configuration
• A broken configuration will cause tests to fail unnecessarily
• A weird configuration might create inconsistent test results.

[mikfreedman]

This is hard to solve immediately due to the way Thor's configuration is specified. We currently read local configuration when specifying our Commands class.