Adds configurable interval and minutes.

Can now specify schedules such as "Execute every 15 Minutes".
Fix to ensure ExecuteInterval defaults to 1 if invalid.
Add test that ExecuteInterval defaults to 1 if invalid or not specified, add test for minutes, add test that it reschedules at custom intervals.
Merge pull request #28 from mediasuitenz/update/add_custom_intervals

Adds configurable interval and minutes.