First rough implementation of the Nepomuk TagProvider.
It is disabled by default, see resourcemanager.cpp if you want to turn it on.

I'm not proud of the current design because:
- TagsFetchJob and TagsStoreJob are assymetric wrt their arguments and return values
- these jobs are in fact synchronous (there seems to be an option to use the Nepomuk D-Bus interface)
- I should have hidden more details behind PersistentFeedFetchJob, but I can't decide on its usage (see helperjobs/)
- I had to make the 'uri' of a Tag public
- the TagProvider is not hidden from the library user

Nevertheless, I commit it just to have a break from Nepomuk