New feature: set, Improvement: contact pivotal only on demand

I mostly use pf start only to set my current local ticket, but i have to wait for pivotal to get a response, which sometimes takes a while. Just wanted to make a little feature `pf set 123123` which will only update local file.

Also - there's need to contact pivotal only if you actually want to fetch something from it, so I've added a little improvement, which will do that.

Cheers