Fixes broken datetime xslt extension. (fixes #20)

[mied]

This should fix the broken implementation.
Now my test file (see issue #20) doesn't fail anymore and outputs this:

<out xmlns:date="http://exslt.org/dates-and-times">
  <datetime>2015-02-04T17:41:29Z</datetime>
  <date>2000-01-02Z</date>
  <time>03:04:05Z</time>
  <year>2000</year>
  <leapyear>true</leapyear>
  <monthinyear>1</monthinyear>
  <monthname>January</monthname>
  <monthabbreviation>Jan</monthabbreviation>
  <weekinyear>52</weekinyear>
  <dayinyear>2</dayinyear>
  <dayinmonth>2</dayinmonth>
  <dayofweekinmonth>1</dayofweekinmonth>
  <dayinweek>1</dayinweek>
  <dayname>Sunday</dayname>
  <dayabbreviation>Sun</dayabbreviation>
  <hourinday>3</hourinday>
  <minuteinhour>4</minuteinhour>
  <secondinminute>5</secondinminute>
  <formatdate>Sun, 2 Jan 2000 03:04:05</formatdate>
  <weekinmonth>1</weekinmonth>
  <difference>P1DT3H4M5S</difference>
  <add>2000-01-03T04:05:06Z</add>
  <addduration>P5DT6H7M8S</addduration>
  <sum>P22Y11MT11H</sum>
  <seconds>946782245</seconds>
  <duration>P1DT2H3M4S</duration>
</out>