large refactoring (see details)

major

asset retrieval and publication now on Virtual Repository, Assets
reverted to plain beans;
VirtualRepository functionality now in terms of
discovery/retrieval/publication;
SPIs matches new terminology: Repository is RepositoryService, Reader is
Importer, Writer is Publisher, fetch() is retrieve(), write() is
publish(), etc;
Reader#discover() now is Browser#discover(types) (there are multiple
readers after all!!);
allow checked Exceptions in SPI;
renamed Wrappers into Adapters, accessible only through factory methods
discovery tolerates interaction failures (discover what it can);
discover() returns number of newly discovered assets;
discover() parallelises interactions with a CompletionService and uses
timeouts;
retrieve() and publish() use timeouts (based on Executor); 
tailored parametrisation of RepositoryManager to use from
VirtualRepository;
RepositoryManager now package protected, used only by
VirtualRepositoryImpl;

minor

realigned documentation, adding more details;
discover() logs a more complete reports with newly discovered,
refreshed, and total assets;
retrieve() and publish() log in detail
tested incremental discovery
re-aligned tests facilities to novelties
added more tests