use opt_fields to avoid one /task/ api request per task

Fetching tasks was taking forever. I thought my nginx/fastcgi was at fault (buffering etc). After digging into the source however, I learned I was barking down the wrong tree; we were retrieving task details with a API request on a per-task basis! Since I'm quite far from Asana's servers, plus I have a biggish workspace, so I was hit pretty hard.

Some googling led me to the `opt_fields` parameter [1], allowing us to re-use the task listing API request to obtain details on all tasks.

[1] http://stackoverflow.com/a/11660287

Also skips checking of assignee (see message of 118d398 for reasoning).