Integrate with time tracker harvestapp.com

Description

We use harvestapp.com for tracking our time. It would be very nice to have integration with Harvest in Gitlab.

Specifically I would like to be to create, start and stop timers from a Gitlab issue.

When Gitlab has this, I might be able to convince my coworkers to stop using Trello cards and instead use the Gitlab issues. Which I look forward to.

Proposal

Links / references

API documentation: http://help.getharvest.com/api-v1/timesheets-api/timesheets/adding-updating-time/

Documentation blurb

(Write the start of the documentation of this feature here, include:

1. Why should someone use it; what's the underlying problem.
2. What is the solution.
3. How does someone use this

During implementation, this can then be copied and used as a starter for the documentation.)