Datepicker: allows the correct use of 0-99 years,

Fixes #7098, http://bugs.jqueryui.com/ticket/7098.
The cause of the bug is relatively simple, javascript date constructor cast 0-99 years to 1900-1999. In order to fix this I had to introduce the "_getDateObjectWithFixedYear" function and change all the "new Date(year, month, day)" calls to this new function.
Also, this fix only applies when "shortYearCutoff" is setted to 0.