Adds Harvest Tracker Support

Adds support for sending time to Harvest Tracker (http://www.getharvest.com/) after each pomodoro.

### How to use:
- Configure tracker in .pomorc like so:
```yaml
:tracker:
  :domain: your_harvest_domain
  :username: user@email.com
  :password: harvest_password
```
- Pass in the harvest project name and task type name you want to log your time against when creating a pomo task.