Implement XSL extension.

TODO:
  - allow SimpleXMLElement objects for importStylesheet.
  - allow Node arguments to registered PHP functions.
  - allow multiple parameters for setParameter.
  - implement setSecurityPrefs/getSecurityPrefs.
  - consider namespaceURI for parameters.
  - add EXSLT support.

Signed-off-by: Arthur Loiret <arthur.loiret@ens-lyon.fr>