Time support plugin

WIP :construction: 

This PR:

* adds the ability to register plugins with the date range picker - using the same basic plugin architecture from the Tabler codebase.
* adds a time support plugin which when enabled renders a panel within the date range picker and allows the user to specify a time for each calendar

![time-support-plugin](https://cloud.githubusercontent.com/assets/1387956/4353456/5b4c0f16-4233-11e4-99fd-2e2df9bb00c4.png)

__Upcoming updates__
* add a checkbox to the panel to allow the user to opt into specifying time (see option 2 here: http://2plzrl.axshare.com/#p=new_page_1)
* add field labels
As discussed, I think the following is simpler:
```javascript
picker.$el.find('[name=specifyTime]').prop('checked', true).trigger('change');
```