Fix: Compilation error on XSLT with includes.

A null XmlUrlResolver was being passed to Load() which prevented
includes from being processed. Fixed by passing a new XmlUrlResolver(),
but we still pass null when UseTrusted==true because external XSLT files
are not trusted.
Fix: Compilation error on XSLT with includes (previous fix not quite correct).
Merge pull request #145 from colgreen/master

Xslt task failed with XSLT compilation error