Expect path/from attributes also as JsonPoiner instances (#60)

[stefankoegl]

Expect the path and from attributes of patches also as JsonPointer instances.

As described in #60, if a user has these paths already in a pointer object, it needs to be serialized to a string just to be re-parsed within JsonPatch.

This avoids the unnecessary roundtrip and lets the user create patches like

    pointer = jsonpointer.JsonPointer('/foo')
    patch = jsonpatch.JsonPatch([                                           
        {'op': 'add', 'path': pointer, 'value': 'bar'},
    ])