Add interaction with Harvest services

I've added functionality to Hubot that allows him to interact with the [Harvest time tracking API](http://www.getharvest.com/api/time-tracking) (general info about Harvest is [here](http://www.getharvest.com)).

The plugin enables Hubot to do the following:

* Report today's timers.
* Report available project/task combinations
* Start a timer on a project/task combination
* Stop a timer

Valete,
Marvin