Container option

This adds the `container` option known from [core boostrap js plugins](http://getbootstrap.com/javascript/#popovers-usage). 

**Usage**
```javascript
$(selector).datetimepicker({ 
    container: nativeDOMElement  || jQueryObject || jQuerySelector
});
```

The new option tells the plugin to create the picker in the specified `container` instead `body`, allowing for e.g. pickers that scroll with a relatively positioned `container`. `container` defaults to `'body'`, maintaining backwards compatibility. 

Additionally this PR includes a minor fix where the picker was placed slightly off when `pickerPosition: bottom-left` or `pickerPosition: top-left` was specified.