Add Asana service

#### 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:
![](http://i.imgur.com/3oZ9ct4.png)

In Asana:
![](http://i.imgur.com/sjcCbN6.png)
Line is too long. [98/80]