new meta data field for erg data, duration, to be shown in workout lists
Descriptions now added to file list based tables

...Favourite and Basic workout lists
now generates some pretty rad little previews for the favourite and basic workouts
dont allow users to load ergDB workouts that have no data
now using NSCache instead of mutable dict, such a nice class.
Image and metadata files now loaded asynchronously

This was done to reduce slow display of file based workout lists. Other optimisations have also been included in this commit