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.