Bug 845173 - [MMS][Utility] smil document generate and attchment extract utility for MMS message

This is only a interface and brief structure for smil document handling with current message app. We will separate the mms message message content into smil slides by this way:
```html
<!--Each slide page could only contains one media area and one text area at most bubble content example -->
<img ....  / >  <!-- slide 1 : page  with image and text -->
<p> string1 </p>

<img ....  / > <!-- slide 2: page only contain one image -->

<video ....  / > <!-- slide 3: page with video and text  -->
<p> string3 </p>
```
And please convert these information to slide array like this:
```javascript
var slideArray = [
{name: img1_name, blob: img1_blob, text: string1},
{name: img2_name, blob: img2_blob, text: null},
{name: video1_name, blob: video1_blob, text: string3}
]
```
All the smil related API will be in smil.js and using SMIL.generate / SMIL.parser for handling mms message.
For sending MMS:
```javascript
     // Using SMIL.generate to create msg object that contains smil document and attachment for sending MMS
     var msg = SMIL.generate(slideArray);
     req = navigator.mozMobileMessage.sendMMS({
       receivers: [number],
       subject: '',
       smil: msg.smil,
       attachments: msg.attachments
     });
``` 
For receiving MMS:

```javascript
     onMessageReceived = function(event) {
        var msg = event.message;
        // Apply SMIL.parse here to retrieving the multimedia source blob, file name and text string in mms message.
        SMIL.parse(msg, function(slideArray) {
          // The structure of slideArray described previously. We should use this slideArray for rendering the MMS 
          // message in the message app page.  
       });         
     }
     navigator.mozMobileMessage.addEventListener('received', onMessageReceived);
     });
``` 

I already added some simple logic for showing the mms with image attachment in this patch. If you have any other question or feedback please contact me ASAP, thanks!