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...
Merge pull request #2046 from johnnyreilly/master

jQueryUI: started tidying datepicker