Add check for PHP XSLT Processor extension.

When the PHP XSLT Processor extension is not loaded, Islandora will
silently fail when attempting to add new objects.  This changeset
adds a check in the Islandora hook_requirements() hook for the
XSLTProcessor class.  If it isn't found, a message is thrown.

Conflicts:

	islandora.install
Merge pull request #278 from nigelgbanks/7.x-class-check

Add check for PHP XSLT Processor extension.