Allow scripting processing to be enabled on a per-page basis

Most users will not need scripting support for the documents they write. When a new document is created, Velocity and Groovy processing should be initially disabled. Provide a way for the user to enable Velocity and/or Groovy processing for the page, if needed (perhaps as described in XWIKI-190 http://jira.xwiki.org/jira/browse/XWIKI-190).

This will significantly boost XWiki's usability for new users, since they are less likely to create documents that will cause processing exceptions. It will also boost performance, since most pages will not require the overhead of Velocity / Groovy processing.