Allow modification of AGraph attributes when copying

[rossbar]

As noted in #436, the AGraph.copy method creates new graphs that preserve attributes from the original graphs that can not otherwise be overwritten: the name attribute, for example.

>>> A = pgv.AGraph(name="test")
>>> B = A.copy()
>>> B.name == A.name
>>> B.name = "testB"
Traceback (most recent call last)
   ...
AttributeError: can't set attribute 'name'

The copy method should support renaming the graph instance. Similarly, there may be other read-only attributes of the AGraph instance that should be allowed to be modified when copying.

See #436 for further discussion.

[dschult]

It's actually slightly different (worse?) than that. B=A.copy() means that B has the empty string as the name. It is also strict even if A is not strict. Basically, B is created with the default attributes (except for directed which is copied over) and then all the nodes and edges and graph_attr values are copied over. I guess we should allow the static and name attributes to be copied over by default and maybe morphed during that process.