Enable multi accounts

PivotalTracker plugin for omnifocus.rb is awesome. Only one point it lacks is support for multi accounts. I'm using PivotalTracker both for work and for personal tasks, so I tweaked this plugin a bit and added.

Config .yml should be look like the one below.

```
$ cat ~/.omnifocus-pivotaltracker.yml
---
- 
  :token: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
  :user_name: muo
- 
  :token: bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
  :user_name: KN
```

Maybe someone would want "projects prefixing" feature for avoiding stories in different accounts merged into one when they have same project name, but it'd be another issue.