Support for partial synchronization (redmine 705)

[prioux]

The new Data Providers store each userfiles in a private directory in the caching system. This could be used to implement selective caching of a userfile's content: for instance, just the HTML and JPG files of a CivetCollection so that they can be displayed. The synchronization layer could record this with the special keyword 'Partial', which in most situation would be interpreted as the same as 'ProvNewer'. The cache subdirectory could contain a special file with a list of the elements partially synced.