Add WebRTC video chat feature for win meterpreter

This PR adds a video chat feature to the Windows meterpreter via Web Real-Time Communication (WebRTC). It is related to the following ticket ("Allow the attacker to interact with the target (human) via a webcam"):
http://dev.metasploit.com/redmine/issues/8763

### Test Requirements:

In order to test this PR, you must prepare the following items:

- [x] A webcam for the attacker.
- [x] A webcam for the victim (VM)
- [x] A Windows VM (victim)
- [x] On the attacker side: One of these browsers that supports WebRTC: Chrome (v23 or newer), or Firefox (v22 or newer).
- [x] On the victim's side: One of these browsers that support WebRTC: Chrome (v23 or newer), or Firefox (v22 or newer)

### Verification steps:

- [x] Have the Windows VM up and running. And make sure a webcam is connected to it.
- [x] Make sure you, as the attacker, are connected to a different webcam.
- [x] Get a Windows meterpreter session going.
- [x] Under the meterpreter prompt, issue command ```webcam_chat```
- [x] Confirm that you see a video session going.

### Before you ask any questions, see the following:

* Recording is currently not possible due to API limitations with Chrome.
* In the attacker's interface, after you allowing the webcam to run, it will try to ask you again even though the video chat is already running just fine. Ignore that. After the first prompt, you should just press 'X' to ignore the rest. Or you don't really have to do anything.
* Internet Explorer does not support WebRTC natively.
* If you work with me and need another webcam to test, please come find me.
* Yes, you can full-screen the video if it isn't big enough for you. Volume control too.
* No, I'm not an UI expert, don't judge me.

### Demo from the attacker's view:

![vid_session_example](https://f.cloud.github.com/assets/1170914/2145733/8b9fa5b8-93ad-11e3-963b-e7e2255cbf8c.png)