now reading attributes from GEXF and storing them in the meta property
Refactored space API, with versions. Breaking.

Briefly:
 - Space no more contains graphs and metas IDs in the client, but returns the number of graphs instead.
 - No more api/graph/last routes, replaced by api/space/graph/:spaceId/:?version routes.
 - Added light comments on every routes.

BUT the whole front is broken right now...
adapted front to last backend refactoring