getExternalIceServers added. connection.mediaConstraints updated.

`connection.getExternalIceServers` boolean added. Now, by default,
RTCMultiConnection gets TURN/STUN servers from xirsys however you can
disable this feature using same boolean
object.

```javascript
connection.getExternalIceServers = false; //
disable
```

`connection.mediaConstraints` and `connection.media`
updated. Now it works same like getUserMedia
API.

```javascript
connection.mediaConstraints = {
    mandatory: {

maxWidth: 1280,
        maxHeight: 720,
        minAspectRatio: 1.77

},
    optional: []
};
```

`connection.media.min` and
`connection.media.max` sets min/max width/heigth in the
`connection.mediaConstraints.mandatory`
object:

```javascript
connection.min(1280, 720);
connection.min(1920,
1080);

// it is actually similar like this;
connection.mediaConstraints
= {
    mandatory: {
        maxWidth: 1280,
        maxHeight: 720,

maxWidth: 1920,
        maxHeight: 1080
    },
    optional:
[]
};
```

You can use SD video like
this:

```javascript
connection.mediaConstraints = {
    mandatory: {},

optional: []
};
```