discussed in #307

Summary of changes:

 * added methods ```add_graph``` and ```remove_graph``` to the Store
   API, implemented these for Sleepycat and IOMemory. A flag,
   ```graph_awareness``` is set on the store if they methods are
   supported, default implementations will raise an exception.

 * made the dataset require a store with the ```graph_awareness```
   flag set.

 * removed the graph-state kept in the ```Dataset``` class directly.

 * removed ```dataset.add_quads```, ```remove_quads``` methods. The
   ```add/remove``` methods of ```ConjunctiveGraph``` are smart enough
   to work with triples or quads.

 * removed the ```dataset.graphs``` method - it now does exactly the
   same as ```contexts```

 * cleaned up a bit more confusion of whether Graph instance or the
   Graph identifiers are passed to store methods. (#225)