Add Asana service

Created by: j0k3r

Current state: WIP - Do not merge

• working asana communication
• ability to restrict to Branch (like the master)

What does this MR do?

When someone commit with a message including

• "blabla #123456", it will attach a new message to the task 123456 in Asana.
• "fix #456789", it will attach a new message and close (mark as done) the task 456789 in Asana

Also, I've added ability to render service.help as markdown. It's more easy to write the help message then.

Are there points in the code the reviewer needs to double check?

• app/models/project_services/asana_service.rb
• app/views/projects/services/_form.html.haml

Why was this MR needed?

We use Asana as our Issue tracking system. It will ease the relation between Asana and Gitlab by linking Asana tickets to git commit.

What are the relevant issue numbers / Feature requests?

Nope.

Screenshots (If appropiate)

From the Gitlab service page:

In Asana: