Add 'auto-discover existing graphs for multi-graph mode

To simplify the configuration process (removing as much as possible spring configuration for end-users), this is a proposal for a "auto-discovery" multi-graph mode in GraphServiceImpl. 

This mode, once activated, just load all graphs under the base resource directory. The user do not have to specify anymore a list of graphs to load in the spring configuration file. The new auto-discover mode is compatible with the existing "static list of graphs" mode.

This also allow the server to be reloaded w/o loosing the configuration. In the current implementation, to dynamically add a new graph you need to add the graph on disk and call a web-service to load it. Unfortunately, if you forget to modify the spring config (depending on the config this also may need the recompilation of the war), when the server is restarted this new graph will not be loaded any longer.

Comments welcomed.