Set position

I needed to change the width of the panel at different breakpoints, so I added a function to the API to let you do that on the fly. 

Simple Usage: 

```js
jPM.position('258px');
```

Usage with jRespond: 

```js
var jRes = jRespond([
  {
    label: 'narrow',
    enter: 0,
    exit: 959
  },{
    label: 'wide',
    enter: 960,
    exit: 10000
  }
]);
jRes.addFunc({
  breakpoint: 'narrow',
  enter: function() {
    jPM.position('258px');
  }
});
jRes.addFunc({
  breakpoint: 'wide',
  enter: function() {
    jPM.position('362px');
  }
});
```