docblock fixes
Support for "default" graph in GraphStore

The following methods of EasyRdf_GraphStore:

* `get`
* `replace`
* `insert`
* `delete`

can accept `EasyRdf_GraphStore::DEFAULT_GRAPH` constant value in $uriRef parameter. This will lead to using "default" graph of graph store for requested operation.

Additionally, the following shortcut methods were added:

* `getDefault`
* `replaceDefault`
* `insertIntodefault`
* `deleteDefault`

refs #166, #187
added changelog entry