Add a new json view to get hotfixes

A new json view is added to hotfixes, so that it could be requested
by plone.app.controlpanel for showing information about updates and
maintance support
Added docstring and super call
fixes two errors, that a json response is send if a none existing version is requested
Merge branch 'master' into json_view
Merge pull request #1 from plone/json_view

While looking over open pull requests i found this one. 

I just merge this since it looks fine. 

This package itself needs some love anyway (see #2)