Category page always redirect to first panel.

Fixes #5
Categories hidden when no backup is selected.
Invalid backup can’t be selected.

Tap #12
Added simple cache and info storage to backup instances using JSON
+ Added custom serializer/deserializer “formats”
+ Using legacy json module
+ Key->Value datastore

Tap #13
Plugins now store output in cache. Fixes #13
Ignore virtualenv folder for py2 and py3