UI for change document type

As requested have ported over the pull request that was pending on Codeplex here:
https://umbraco.codeplex.com/SourceControl/network/forks/abutland/WorkingFork/contribution/4076

It's an implementation of a UI for changing of a piece of content's document type, as referred to in issues U4-79 and U4-1468 and on the development mailing list topic "U4-79/U4-1468 UI for Change Document Type". 

It uses the V6 API, with some additional checks and calls for document type validity in the content structure, mapping of properties and selection of an appropriate template.

It's set up with translations and user permissions.

Note that in addition to the code there needs to be one small database data update, to give the administrator user type the permission to carry out this action - as follows.

UPDATE umbracoUserType 
SET userTypeDefaultPermissions = userTypeDefaultPermissions + '7'
WHERE Id = 1

Hope it proves useful.