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)