New attachToIssue and assign methods

Hi, I've added the attachToIssue method to upload an attachment to a JIRA issue. 

Unfortunately I haven't added any tests as there's a nested callback that I'm not able to test properly. I'm just a noob on the Node.js testing matter, sorry. 

On the other hand, it works perfectly in practice.

Edit: I've now added the assign method for assigning issues to users