Add option to accept override for properties of the DPGlobal object

To use this option, declare the `templateOverrides` option as a space-separated list of values - which match a property of DPGlobal, and *also* re-declare that property as an option,  like this

```javascript
.datepicker({ 
    'templateOverrides' : 'headTemplate contTemplate',
    'headTemplate' : '<foo></foo>',
    'contTemplate' : '<bar></bar>',
});
```