Add calendar exporter

Using the `courses` command with the `--export` option, the course schedule is exported as an iCalendar (.ics) file in the current directory.
Maybe instead of

``` ruby
schedule = schedule(semester, year)
schedule.fetch_courses
```

we can just use

``` ruby
schedule(semester, year).fetch_courses
```

I think it's a bit confusing with schedule being used as a variable and method name.