Add EditPage help / $smwgEnabledEditPageHelp

Finding help entry points is not always easy for novice user
therefore displaying help on the edit page will help those
yet unfamiliar with SMW.

Depending on the type of page, messages will change. Setting
$smwgEnabledEditPageHelp = false will surpress all smw-related
message from the edit page.
Merge pull request #659 from SemanticMediaWiki/editpage-help

Add EditPage help / $smwgEnabledEditPageHelp