SpaceIndex macro:adding some sorting parameters

Improve SpaceIndex macro:adding some sorting parameters
Cf.: http://jira.xwiki.org/browse/XCOMMONS-602
I add the sort order parameter:
Name: sort
Optional: yes
Allowed values: creationDate, modificationDate and name
Default value: creationDate
Description: creationDate sorting the list by creation date of documents, modificationDate by update date and name sorting alphabeticaly the list

Example:
spaceindex space="Sandbox" sort="name" /
return the list of Sandbox space alphabeticaly ordering