jQueryUI: started tidying datepicker

Refactored jQuery UI datepicker overloads and added JSDoc. Options still to do - will follow in a later commit I hope.  

Removed unused optionLiteral datepicker overloads (undocumented).  Made the following change to the underlying methodName overload to allow other overloads to "chain" into it.

```
datepicker(methodName: string, ...otherParams: any[]): any;
```

Moved datepicker tests to single function.  Added tests for datepicker methods.

Very much just first steps.  A lot to do in jQuery UI I think...