Refactor icalendar helpers into their own module

[ryannjohnson]

This is a step towards resolving #836.

I'd created a function in khal/utils.py that I wanted to use in khal/parse_datetime.py, but I couldn't because of the circular dependency that would create.

# khal/utils.py

import khal.parse_datetime as parse_datetime  # TODO get this out of here

I saw this TODO comment and decided to just go for it.

---

I've additionally included a small fix for the failing test in tests/parse_datetime_test.py. It now parses string categories on khal's side instead of letting icalendar try to iterate through them.

Adding categories via khal new --categories a,b,c ... still seems to be broken, same as before.

[geier]

Thanks for taking the time to improve khal and sorry for my inactiveness. The refactoring looks sensible and lots of good smalf fixes, if you could rebase it against master, I would merge.

[ryannjohnson]

Done!

[geier]

Thank you!