Eliminate redundant calls to `postProcess()`

In **Release 4.1.0**, lines [1274-1276](https://github.com/kbwood/datepick/blob/master/jquery.datepick.js#L1274) were introduced to handle redundant calls to the `postProcess()` function.  However, looking 'upstream' a little more to diagnose why redundant calls were happening in the first place, I realized that the only reason I could see for the lines **1293-1295**

    if (!showAnim) {
      postProcess();
    }

was to handle the case that in older versions of jQuery (for example 1.4.4) the callback would not be executed if a valid option was not given for the `jQuery.fn.hide` method's `speed` parameter.   The parameter `''` is not a valid option.

Changing the fallback parameter in line [1291](https://github.com/kbwood/datepick/blob/master/jquery.datepick.js#L1291) from `''` to `0` eliminates the necessity of lines **1293-1295**, which in turn eliminates the need for lines **1274-1276**, since redundant calls will no longer occur.

I've verified basic functionality for jQuery 1.4.4, 1.5.2, 1.6.4, 1.7.2, 1.8.2, 1.9.1.

BTW, if you'd like to make a **dev** or **working** branch for me to submit the PR to, I'd be happy to resubmit.