Display the conversation name in title/notifications for increased usability when running multiple Cryptocat tabs

Related to ticket #454, I've made the following modifications, which I feel help with usability when using multiple simultaneous Cryptocat sessions.

 1. Display ' @ conversationName' in title of desktop notifications when a message is received, rather than just the name of the sender.
 2. Include 'joined' or 'left in desktop notifications when someone joins/leaves a chat, rather than just their name.
 3. Set the window title to 'myNickname@conversationName' after a successful connect.

I found these tweaks improved matters when running multiple conversations at once, so I thought I would submit them for possible inclusion.