ActiveCollab improvements

The two changes are:

- pull in comments as annotations
- use the [pyac](https://github.com/kostajh/pyac) library for calling ActiveCollab

I did my best to make sense of the annotations framework by looking at the Github, Jira and Trac services. What I have is very close — if you add `pprint.pprint(extra)` at line 259 of activecollab.py, you'll see the annotations are there, but they are not getting saved with the task. @coddingtonbear, if you have a minute can you look and see what I am missing?

Also by removing `call_api()` in favor of pyac, I broke the test, but I don't know how to fix that.

So to summarize, before merging, this needs:

- [ ]  tests to be fixed
- [ ] annotations to save properly