[NFR] set a class to syphon non-form elements

As contenteditable gets better and its use grows, there might be [more demand](http://stackoverflow.com/questions/18854141/using-non-input-elements-with-backbone-syphon/32137166#32137166) for non-form extraction.

This could be a simple change.

``` js
return viewOrForm.$(':input');
```

to 

``` js
return viewOrForm.$(':input, [contenteditable], .syphon-editablecontent');
```
